NOTE: Blackmorgirm is working on a Summary/TL;DR of this post. It will lag behind slightly, but that won't stop me from posting this now. I'll prepend it to this post once he has had a chance to finish constructing it.
These thoughts are from a Heroic and Mythic PvE raiding perspective. Where appropriate, I try to tie back in how various aspects affect broader groups (such as Dungeon/Normal raiding or PvP as opposed to strictly Mythic raiding). There are many more specific topics, some even being favorite pet peeves within the Shadow community, which could be discussed at length but have been omitted or truncated. There are always topics available to dive in to at depth. For this post I've settled on Talents, Glyphs, Stats, Tier Set Bonuses, and the Hellfire Citadel Class Trinket. Many other topics are discussed inline (to varying extents) including resources, ability synergy, movement, baseline mechanics and their shortcomings/benefits, and others. This isn't to say that there aren't both good and bad things with these other topics that aren't worth discussing, just that they are not included on their own. One large piece that was purposefully limited and in many cases left out entirely are suggestions for how to adjust or fix things -- that's a whole other unwieldy and large topic that I'm positive will happen in the comments.
Breaking Down Talents and Glyphs
There are several talent rows that are quite well balanced and all have distinct homes in PvE. These talent rows are largely those that do not affect DPS rotations and instead provide some sort of secondary ability or effect that may help with fight mechanics or survivability. While this can't be the only goal behind a talent row, having meaningful and viable options available is key.
In general, talent rows that conform to a central theme help with immersion and fun. While this may not always be possible due to balancing or limits on mechanics (there are only so many ways you can achieve a similar affect before they all start being too similar), this is one area that seems to be lacking in some talent rows, specifically the DPS talent rows.
Level 15 Talents
This talent row is a prime example of a good mixture of choice and personal preference, all of which boil down to avoiding or recovering from damage. Spectral Guise has many great advanced uses to ease encounter mechanics; Desperate Prayer is an instant and large percentage heal for when you need a boost and have time to react to it; Angelic Bulwark gives the Priest a good passive shield (that is less effective, raw Health wise, than Desperate Prayer) that can prevent the Priest from taking fatal damage when you don't have enough time to react.
The talents are all themed around damage prevention/recovery in crucial/low health situations. Every talent has a logical use but doesn't limit you to always take one because it is 'better' than the others. This is an example of an excellent talent row.
Level 30 Talents
This talent row has two out of three options that are very good, and a third that doesn't have a place in PvE. Body and Soul piggybacks on Power Word: Shield to provide increased speed for a short period of time while simultaneously granting the damage absorption; Angelic Feather grants increased movement speed more often and for a longer duration; Phantasm prevents you from being slowed and removes any current slows on you.
The talents are all themed around two things: 1) movement speed, and to a lesser but still significant extend 2) reduction of damage by getting out of harm's way or mitigating it. This talent row is once again much about personal preference. The entire point of being able to move quickly is to get to a safe place/out of a dangerous spot. Body and Soul trades some movement for damage absorption (damage taken being what you're trying to avoid, ultimately) while Angelic Feathers lets you or others move for a longer distance to get out of harm's way (which, depending on the mechanic, may mean you take less damage overall than from a Power Word: Shield + movement via Body and Soul) and can be set up in advance, removing the rampup GCD when you really do need to move in a hurry and can predict it.
Phantasm is a bit out of place here. While it doesn't increase your movement speed above base (only back to), it also does not aid in reduction of damage. In every situation encountered during a raid where removing slow affects on you would be helpful to avoid taking damage, either a healer will dispel it (and is already looking for it to go out on a raid member) or it is the Shadow Priest's job to Mass Dispel, it can be trinketed (if you are a Human), the area you need to get out of isn't large enough / the effect is on enough of a delay that Body and Soul or Angelic Feathers will give you enough of a boost to move out of, or Dispersion or Glyph of Fade + Power Word: Shield is enough to absorb the damage you would take from failing to move.
For this talent row, two out of three are very good and gives a lot of choice. Phantasm, while fitting with the overall theme, does not offer anything extra. For a brief period of time, the 6.1 patch notes included a change to Phantasm that would make it viable in raids: add a damage reduction component as well. This is almost the same effect as Glyph of Fade, though, so likely only one could stay.
This is an example of a very good talent row.
Level 45 Talents
This talent row has a "default" talent pick and two others that rarely see use. Surge of Darkness grants procs based on Vampiric Touch and Devouring Plague ticks (via spent Shadow Orbs). Insanity grants increased single target damage and AoE damage when you spend Shadow Orbs. Mindbender is an amped up version of Shadowfiend.
The primary theme that is seen in two of the three talents in this row is Shadow Orbs granting new and interesting effects that affect how you do damage. Mindbender is the lone talent in this row that does not interact with Shadow Orbs at all. If you pick Mindbender, there is no managing of Shadow Orbs to be done (sans tier set bonuses altering playstyle).
For this tier, let's look at each of these talents in respect to the Level 100 talents, since those affect the playstyle of each (some more significantly than others):
Clarity of Power
Insanity is the default choice the majority of the time because it is not dependent on any randomness, and, it is controllable burst. This is key because of DoT Weaving, where you bank 5 Shadow Orbs, apply Shadow Word: Pain and Vampiric Touch (instead of using 2 GCDs of Mind Spike), then begin spending Shadow Orbs on Devouring Plague followed by Insanity twice back to back. There are many different versions of this depending on a player's Haste level and tier bonuses, but it is a slight DPS gain (~2%) in single target situations over casting Devouring Plague and using Insanity whenever it is available. This extra potential damage also comes at a large risk of DPS loss: if a DoT Weave rotation is botched by the player, either by removing DoTs from the target early with a Mind Spike, the target dying/becoming immune/out of range, or raid events interrupting it, the gain of DoTs vs. Mind Spikes for the two lead-in GCDs will be a damage loss. The core issue with DoT Weaving is that it goes against the rest of CoP from a stat priority point of view because less damage is affected by Mastery than with the intended CoP rotation due to the introduction of DoTs to a single target rotation. This leads to players building sets that maximize their DoT Weaving stats rather than focusing on Mastery; the end result is that most players who aren't playing at the top levels do less damage when they inevitably make a mistake in the play style and go back in to the Shadow Orb building phase that uses Mind Spike and Mind Blast exclusively.
Aside from DoT Weaving, Insanity has very good synergy with CoP's increased damage to Mind Sear for those burst AoE situations where there isn't time to apply DoTs. Insanity is also very strong when there are multiple targets that all need cleave damage with one being designated as a primary kill target, or, a new target appears and becomes priority where burst is needed.
Surge of Darkness is not as strong as Insanity in single target (DoT Weaving or otherwise) but grants the advantage of movement DPS if the need arises. This is good for less experienced players who may not be able to schedule their ability usage around predicted/known upcoming movement (where Insanity results in higher damage). Surge of Darkness's downfall is that it has some awkward interaction with CoP once you begin adding additional targets. Since Surge of Darkness has two ways to be proc'd, from Vampiric Touch or Devouring Plague ticks, to leverage additional targets to grant extra procs you must apply VT to some (or all) of these targets at the cost of: A) casting VT on all targets and using Mind Flay instead of hardcasted Mind Spikes on the primary target to avoid removing Vampiric Touch, and, not benefit from the bonus damage on Mind Spike from CoP, or B) only apply VT (and SWP) on the secondary targets and continue to use Mind Spike on the primary target as a filler. The design of CoP implies that DoTs should not be applied to your primary target, however, in many cases Priests achieve higher damage in this situation by applying DoTs to all targets instead of just secondary. This makes it easier to switch which target is considered your priority and, depending on how much movement is involved, results in higher DPS than having one target without any DoTs at all.
An additional concern exists for single target where SWP/VT will not be applied to the target. The proc rate of SoD from DP, when accounting for the damage of a SoD Mind Spike and DP ticks, is so low that it is better to remove DP with a hardcasted Mind Spike before it has been able to complete all of its ticks than it is to switch to Mind Flay while DP is on the target and wait for the ticks to complete. This is logically backward from how the talents are designed to interact with one another and is indicative of a damage balance and/or proc rate balancing problem. In 6.2 with the nerf to the damage that DP does overall (which affects how much damage it does per tick), this problem will become further entrenched. The damage, on average, of a SoD Mind Spike proc + Mind Flay as a filler + the tick damage from DP must be larger than the damage multiple (usually 2, sometimes 3) hardcasted Mind Spikes will do. Avoiding mechanics for a talent because playing the intended way is inferior to playing in what would usually be a haphazard and novice fashion is not fun.
Mindbender does not interact with CoP at all. It is merely a button that can be pushed three times more often. It's only advantages are that it gives the players an extra button available to press while on the move, and, there aren't any buff uptime or procs that can be wasted/overflowed if you have too many other things to do (such as refreshing DoTs on many targets).
Auspicious Spirits
Insanity is the default choice the majority of the time because of the constant (at high Crit levels) flow of Shadow Orbs to spend on Devouring Plague. Mind Flay is already the default filler spell for Auspicious Spirits so there is very little change in rotation when the buff is active more often thanks to having additional Devouring Plagues cast. When you have many targets where one is not the clearly defined kill target, there may not be an opportunity to cast Insanity as most of your time will be spent refreshing Shadow Word: Pain and casting Devouring Plague to avoid capping on Shadow Orbs. Searing Insanity is also able to see higher uptime as well, though how useful this is depends largely on fight conditions (target spread, how long they are alive, how often it occurs, etc.).
Surge of Darkness sees a substantial boost in situations where there are a few permanent targets to keep DoTs up on. If you have 3 or more targets, it may be difficult with high levels of Crit to spend your SoD procs due to needing to refresh DoTs on all available targets and cast Devouring Plague to avoid orb capping. As with CoP above, SoD excels when there are heavy movement phases to an encounter.
Mindbender does not interact with AS at all. It is merely a button that can be pushed three times more often. It's only advantages are that it gives the players an extra button available to press while on the move, and, there aren't any buff uptime or procs that can be wasted/overflowed if you have too many other things to do (such as refreshing DoTs on many targets).
Unlike CoP, AS is a spec where it is expected that you will not have many free GCDs to spend on SoD or casting Insanity, so having a "fire and forget" button is acceptable because there is no proc or buff to be wasted.
Void Entropy
Insanity is the default choice for a few key reasons. Due to Insanity being triggered off of any spent Shadow Orbs, not just Devouring Plague, you are able to apply VEnt to a target and still get a benefit from Insanity. While it should not occur more than a couple of times per encounter, VEnt has a slightly higher Insanity uptime per Shadow Orb spent ratio than CoP or AS because you are not GCD locked after casting VEnt (as it hast a cast time, whereas DP is instant) which results in an extra 2 ticks of Insanity. As more targets get added that you must maintain VEnt on, it becomes more and more difficult to save Shadow Orbs for burst -- you have a relatively small window where if you don't cast DP, VEnt will fall off of your target. This is also problematic for any required burst AoE from Searing Insanity.
Surge of Darkness's viability is diminished due to Shadow Orb spending competition between DP and VEnt. While a player may have increased proc chances from applying VT to these extra targets, there will be the lack of extra GCDs spent casting DP that AS has in the same situation. For the rampup period, when damage is already lower because VEnt is not out on all targets yet, will be even lower because of not having DP cast to grant extra SoD procs.
Mindbender does not interact with VEnt at all. It has an advantage over SoD in that the damage it does is not restricted by what abilities Shadow Orbs are spent with. As with AS and CoP, it gives the players an extra button available to press while on the move, and, there aren't any buff uptime or procs that can be wasted/overflowed if you have too many other things to do (such as refreshing DoTs on many targets or the inopportune casting of a DP to refresh VEnt).
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Overall, to summarize:
The L45 talents are a bit of a grab-bag of effects that have varying usefulness depending on what Level 100 talent you choose. Ultimately, Insanity is the go-to talent because of it's high uptime for CoP and AS (due to increased Shadow Orb generation) and for VEnt as well (because the spending of Shadow Orbs on VEnt instead of DP does not reduce the effectiveness). Surge of Darkness is well suited to multiple target situations for CoP and AS but not very much for VEnt. Mindbender is out of place in the scheme of things because it is not affected by neither Shadow Orbs nor any of the Level 100 talent, either directly or indirectly.
This is an example of a talent row that has balancing problems and a lack of choice induced by affects from another talent row.
Level 60 Talents
This talent row has a few niche choices that are heavily reliant on encounter conditions in PvE. Void Tendrils AoE roots, Psychic Scream AoE fears, and Dominate Mind charms.
This talent row is centered around CC. Having the choice between AoE roots and an AoE fear is nice and handy in a raid or dungeon environment, but Dominate Mind is just not fun (in PvE). Any time a player loses control of their character there is a high potential for unexpected, and potentially fatal, things to occur -- especially in a raid environment. This is amplified for a mechanic like Dominate Mind where you are unable to control your own actions at all due to literally controlling another character. With that being said, it is a staple and identifying ability in the Priest class's kit and very valuable in PvP. If there was a way that it could be more engaging without the large risk involved, perhaps as a taking control of mobs as pets in PvE so that the player can retain control over their own character (possibly silenced for the duration / breaking Dominate Mind on spell cast) that would remove or reduce the 'not fun' factor.
This is an example of a talent row that has a meaningful choice between two options and a third that is forced based on the encounter design.
Level 75 Talents
This talent row as a defacto pick and is missing a cohesive theme. Twist of Fate increases damage by boosting raw damage done; Power Infusion increases damage by letting you execute actions more quickly; Shadowy Insight increases damage by increasing resource generation in a random way.
The primary theme, though not one that is easily identified, is boosting damage. Twist of Fate accomplishes this by increasing the damage a rotation does passively; Shadowy Insight does this by making the overall rotation occur more frequently (at random); Power Infusion does this though a mix of both by speeding up the rotation and doing it passively (outside of pushing a button). Twist of Fate is the talent that wins out far more often than the others because it is passive, easily triggered, and sees significantly increased uptime whenever there are extra non-permanent targets.
As with the Level 45 talents, let's look at each of these talents in respect to the Level 100 talents, since those affect the playstyle of each (some more significantly than others):
Clarity of Power
Twist of Fate is the default choice. By procing at 35%, ToF is up for a large portion of the fight, most importantly including execute range. The execute phase for CoP is already exceptionally strong thanks to the increased damage to Shadow Word: Death (61% more powerful than a non CoP + non ToF cast). This isn't a necessarily a problem, but, with the high proc threshold it is very easy to toss a single SWP on a low add or off target that is not in execute range and have insanely high uptime on the 15% damage bonus for most of the fight.
Power Infusion has seen extremely limited usage with CoP due to it being a GCD locked playstyle -- there are no passive sources of damage (such as DoTs) to gain effectiveness from the Haste buff, it simply speeds up the rotation and makes it easier to be adversely affected by latency (where as with AS or VEnt, Mind Flay is more tolerant to latency at high Haste levels due to it being a channeled, 2 GCD ability). Additionally, due to Mind Blast having a reduced base CD, the haste bonus doesn't have as large of an effect (though it is proportionally the same as with AS or VEnt).
Shadowy Insight is almost never used with CoP. It has a very low proc rate (5%), meaning at a 1.25sec GCD if you never cast anything but Mind Blast->Mind Spike*3->Repeat, you would have roughly 1.8 procs per minute. Keeping this in mind, Mind Blast already has a reduced cooldown (3 free GCDs to use between casts), of which half of these procs (assuming every 4th GCD is Mind Blast, as it should be) from casting Mind Spike will occur when Mind Blast is already off cooldown; the other half of these procs will occur after the player has already begun casting their next ability and is GCD locked, or, will offset the entire rotation slightly and occurs when Mind Blast has less than 1 GCD (or slightly less) left on CD. The same situations occur when a player has SWP on secondary targets. In the best case, SWP will cause Shadowy Insight to proc while the player is GCD locked from casting Mind Blast or in the first GCD afterward. Otherwise, a proc will occur during the GCD before Mind Blast would come off cooldown anyway or in the GCD before where you are already mid-cast.
Auspicious Spirits
Twist of Fate is the default choice. As with CoP, ToF is up for a large percentage of the fight due to procing at 35%. The main driver behind Auspicious Spirits is leveraging extra targets to generate Shadowy Apparitions from SWP crits, meaning a player is already going to be applying SWP to anything that moves. If the encounter has adds of any sort that come and go periodically, Twist of Fate's uptime will be significantly higher than the baseline 35%.
Power Infusion is rarely used with AS presently and can not compare to ToF in single target situations; however, PI could be a viable DPS cooldown in a council fight (where ToF uptime can't be gamed) when Crit levels from gear get high enough to net consistent SA output. Looking to T18, there may be a possibility of using PI before the Legendary Ring is activated so that a Doppler Effect is in place with SAs flying towards, and striking, targets during the damage window to fuel as many Devouring Plagues as possible.
Shadowy Insight is rarely used with AS presently as the damage increase from ToF far outweighs the extra Mind Blasts and Shadow Orbs gained. At the levels where enough SWPs would be ticking on targets to get consistent procs, Mind Blast is already relegated to a filler ability in between keeping SWP up, casting DP before Shadow Orbs go to waste, using Shadowfiend/Mindbender, and casting your Level 90 talent.
Void Entropy
Twist of Fate is the default choice. The base uptime in single target results in roughly the same damage as Power Infusion (more on this in the next paragraph), but when adds start appearing it can lose its edge to Shadowy Insight depending on how long lived they are, or, how frequently they spawn.
Power Infusion's damage is slightly behind ToF in one target situations and much further behind in two or more target situations. This is because the first use of Power Infusion is either delayed until you have VEnt ticking on the target (or multiple targets), or, is used without any VEnts ticking at all.
Shadowy Insight is a viable option in certain situations. If there are multiple targets (2 or 3, more than this is difficult to maintain VEnt on), SI procs can help to get VEnt up on all targets more quickly, and, maintain it. Additionally, if you have many frequent waves of adds, you can also leverage Glyph of Mind Harvest on these new targets, with the increased cooldown on Mind Blast's effective impact reduced by SI procs, to maintain VEnt on many (3-4) targets.
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Overall, to summarize:
The Level 75 talents are largely one dimensional from a choice point of view. Twist of Fate, on some encounters in T17, can be abused so much that there is a 90% uptime. If this is the desired niche for ToF to fill (high up time when there are many adds coming and going throughout an encounter), then it should not also be the default choice in situations where there aren't targets frequently coming and going. Lowering the threshold % for the proc to occur will lower overall damage in single target but leave the potential uptime in encounters where there are many adds that will die at the presently seen levels.
With the removal of DoT snapshotting, Power Infusion's effectiveness in general is lower than it was during MoP. There are some potential use cases if just the right situation arises (gear + encounter), but these are few and far between. In general, with the Legendary Ring, pairing Power Infusion with Mindbender (from the Level 45 talents) could be quite strong in single target situations, regardless of which Level 100 talent is selected.
Shadowy Insight is not a viable option for CoP due to the small window in which to use it because of Mind Blast's reduced cooldown and being GCD locked. For AS and CoP, the proc rate is too low to be reliable -- during MoP, Shadow's Mastery increased the effective proc rate up to ~8% depending on gear, and, the Legendary Meta Gem plus DoT Snapshotting allowed for more frequent SWP ticks to occur. For VEnt, SI can be a bit of a saving grace in that it helps smooth out issues with having enough Shadow Orbs to maintain VEnt on many targets.
This is an example of a talent row that has virtually no choice due to balancing problems. Some talents work better for some Level 100 talents than others, but in general there is one talent that is head and shoulders above the others.
Level 90 Talents
This talent row has a defacto pick for single target and a defacto pick for multiple targets, but, has a cohesive theme. Halo does damage in a sphere around the caster based on how far away the targets are; Cascade does damage to targets based on the distance between source and end point; Divine Star does damage to targets in a straight line, out and back.
The theme of these talents is all about positioning and distance. For WoD, up until 6.1 Halo was the only choice available except in situations where there was a very high number of targets available, or, you would have a medium number of targets available more often than Halo would be available for use. Since 6.1, however, Cascade has become the overarching choice any time there are more than a single target due to Cascade bouncing to each target twice. This has left Halo as the default single target ability only because Cascade has nothing to bounce to.
Divine Star is out of the loop here. In MoP, it was very competitve due to the offheading it would provide (~15%-20% health to everyone, if stacked, every 15 seconds). With the removal of offhealing in WoD, Divine Star has lost its niche -- the damage was never good (even below Mind Flay from a DPET perspective in MoP). The new (or, rather, back to the original) use case for Divine Star is to use it when there are frequent periods of many adds stacked on top of each other very often. Out side of this very narrow window of use, Divine Star's damage is so poor in single target that it only beats Mind Flay in DPET until you get to high (100%) Mastery levels or medium (~67%) Mastery levels with the Mythic Class trinket. Compared to Cascade (where it is higher DPET than Mind Flay if you are not right on top of the target) for situations where targets are grouped, if there is even a single available target that is out of the group Cascade will win. There is no upside to Divine Star any more.
A general comment about the Level 90 talents: the distance component to each makes them unique abilities in the game, but is also what makes them incredibly frustrating to use. Halo is the worst offender due to it having a very specific maximum potential damage distance window. Without any built in UI to know how far away you are from your target, players are instead forced to use addons that can only make a best guess at the distance away they are when casting. This truly is a guess because there are limited amounts of information, including inconsistent information, to work with in an attempt to know your distance from the target. Some targets measure their hitbox from the middle of the target, others from the edge. Using class abilities that have fixed maximum use distances helps to get a general idea of the distance, however, attempting to peg 25 yards for a maximum Halo cast with this is unrealistic. The UI addition for Disc and Holy on Halo work well enough to give a rough, built-in idea of maximum distance but still suffers from the same issues as addons if it were converted to be used for DPS -- every target's hitbox is different. Compounding the positioning issue even more is trying to execute fight mechanics properly while the player is simultaneously trying to accurately position themselves, all without any accurate indication of if they are close to being in optimal positioning or not. Often times, encounter strategies make it impossible to be in position when using these talents (not just Halo -- all of them).
Divine Star suffers from this issue as well since it has a maximum 28 yard range (24 yards + 4 yard pulse) that, at the maximum distance, may register from addons as being able to do damage but in reality falls short of the target and does no damage at all, effectively wasting a GCD. Cascade is less affected by this as it reaches maximum power and has a 10 yard buffer window of usage, and, doesn't lose out on large amounts of damage for being off by a few yards, unlike Halo. Cascade is also unique in that, unlike Divine Star and Halo, it has to be cast on a target which allows for enforcement of using it only when you are in-range.
This is an example of a talent row that gives some choice to the player but selection is mostly dictated by encounter conditions. The abilities can be frustrating to use effectively as use of third party addons is mandatory and does not solve the source of the issue.
Level 100 Talents
This is a talent row that has a default pick based on how much gear (tier bonuses, trinkets, and item level included) a player has. Many, myself included, view this talent tier's theme as "sub specs" since each greatly affects Level 45 and Level 75 talent choices, changes stat and enchant priorities, and each vary the style of play much more than any other row of talents. An additional secondary theme can be seen as resource (Shadow Orbs) management. Clarity of Power has been described as the "DoTless" playstyle; Auspicious Spirits has passively increased orb generation that scales with targets; Void Entropy plays off of Shadow's heritage as a DoT class by granting an extra DoT.
Clarity of Power
This talent provides a faster paced playstyle where damage done by DoTs (SWP/VT) is limited, and, any preventing bonus damage on certain key abilities from occurring if Shadow Word: Pain or Vampiric Touch are present on the target. To achieve this, Mind Spike is introduced as the filler ability rather than Mind Flay for many reasons, most significant being that Mind Spike will remove DoTs from the target it is cast at, allowing any errant (or purposeful) application. Mind Sear also receives a damage increase on an individual basis based on if SWP/VT are on them. Shadow Word: Death also receives this substantial increase, making Shadow's execute even stronger than it normally is. The second component to CoP is the reduction of Mind Blast's base cooldown, which increases Shadow Orb generation, and making it instant cast, which gives CoP an ability that can be used while on the move (SWP fills this role for non-CoP playstyles).
Combining both of these effects helps to offset the passive damage lost from not having SWP/VT ticking on a target. This is also the source of some core problems with the talent: if there is a situation where you won't be using any of the abilities that receive a damage bonus, there is no penalty for using other abilities instead. This leads to strange situations like DoT Weaving (mentioned in the Insanity section of the Level 45 talents above) and CoP Lite, where the base Shadow playstyle remains identical to how it was in MoP but with a reduced cooldown and instant cast on Mind Blast. For multi target situations, CoP Lite is nearly always more damage overall than off-DoTing secondary targets and having a normal rotation on the primary target. Additionally, having DoTs ticking on all targets makes it easier to switch between targets when priority changes.
These alternative playstyles exist because ability options are not removed or restricted in a meaningful way. As it stands now, players need to know what the different CoP styles are and when to properly use each one to maximize benefit. Choosing the wrong style results in significantly reduced output. As mentioned above about Insanity/DoT Weaving, this is added complexity that does not result in a meaningful enough gain to be worthwile, with the combined downside that if you make any mistakes you end up far behind where you would have been.
The class trinket attempts to address this issue by doubling down on the damage that Mastery abilities do (plus Mind Sear). This may help to address some issues for single target encounters (specifically DoT Weaving, to an extent), however, the trinket will do little to change CoP Lite from being superior in multiple target situations since it increases the damage from both Mind Spike and Mind Flay evenly.
CoP is supposed to rely heavily on Mastery as its favored secondary stat, due to the majority of abilities that are intended to be used (Mind Blast, Mind Spike, and Mind Flay when DP is on a target) are directly increased by Mastery. Having DoT Weaving or CoP Lite as options reduces the percentage of your total damage which has Mastery as a contributing stat by a significant enough amount. This, coupled with Auspicious Spirits and Void Entropy not being reliant on Mastery for their damage, causes players to ignore Mastery in general in favor of other stats (specifically Crit because of AS) and further amplifies the effectiveness of CoP Lite or DoT Weaving rather than regular CoP.
Auspicious Spirits
The meta game for this talent is proper Shadow Orb use, specifically minimizing overflow when nearing cap. This is accomplished by providing a new source of Shadow Orbs that scales specifically with Haste, Crit, and the number of targets available. This is centered, specifically, around casting Shadow Word: Pain on as many hostile targets as possible to fish for Shadowy Apparition procs. At a medium numbers of targets (~3-4), Mind Flay is not ever channeled as there are no free GCDs while keeping SWP/VT on targets, Mind Blast cast on cooldown, spending Shadow Orbs on Devouring Plague before you are capped on orbs, and using your Level 90 talents and Shadowfiend. Above this number of targets (~5-7), Vampiric Touch becomes a filler when all Shadow Word: Pains are not at risk of falling off the target. At very high target counts (7+), Mind Blast may not even be cast as the player will receive more total damage (from Devouring Plague) from SWP than from casting Mind Blast.
This adds a gradual level of complexity and challenge to the talent where good decisions (when to drop certain abilities from the rotation) and high skill are rewarded. The largest drawback, however, is there is no built in notification system in place to track when you will get Shadow Orbs from Shadowy Apparitions spawning. Outside of using third party addons, players won't know how many or when they will receive extra resources to use. This can be confusing to players new to using this talent because of the unpredictable nature of their resources going from very few to enough to use other abilities in a very short period of time.
Void Entropy
This talent deviates from CoP and AS in theme by costing Shadow Orbs rather than increasing the generation of them. Spending Shadow Orbs to apply Void Entropy as a 1 minute long duration DoT means that you are unable to spend those Shadow Orbs on Devouring Plague causing the player to lose out on any specific secondary effects associated with Devouring Plague, such as fewer procs from Surge of Darkness (as mentioned above), less self healing, and reduced T17 4P uptime. The more times you have to reapply Void Entropy, whether it be because of having a new target or it falling from an existing target for a variety of reasons (phase transitions where the target is immune/target is out of range/players are stunned, too many targets to reliably keep VEnt up on) this same penalty occurs. Additionally, VEnt needs to be on a target for a certain minimum amount of time before it will do more damage than spending those same Shadow Orbs on a Devouring Plague. This varies based on the Spell Power coefficient on both Devouring Plague and Void Entropy, and, how much Haste the player has. In 6.2, with the nerf to Devouring Plague, the minimum amount of time at baseline (5%) Haste will be roughly 23 seconds.
Because Devouring Plague refreshes the duration on Void Entropy, though, it is fairly easy to maintain on a single target. Executing a normal rotation will keep VEnt ticking indefinitely. When a secondary target is introduced, it becomes more difficult to maintain VEnt on both targets especially at low Haste levels. With modest levels of Haste, assuming minimal delay in casting Mind Blast to generate Shadow Orbs, it is possible to maintain two VEnts. Above this, however, and without very specific encounter mechanics in conjunction with specific talents and/or glyph selections (see the Shadowy Insight discussion above on Level 75 talents), it is impossible to maintain three or more VEnts outside of execute range.
One key point to keep in mind is that, assuming you do not need to reapply VEnt more than once per target, the longer an encounter lasts for the more damage you get out of VEnt. This causes encounters where during progression VEnt may be an acceptable choice, once farm sets in and kill times drop substantially any damage advantage that VEnt may have had will disappear.
For less skilled players, where attacking more than a single target at a time is less likely to occur, Void Entropy has a strength in its simplicity: the rotation that players learn while leveling largely remains the same, the only difference is that at the beginning they press a different button a single time. In this aspect, Void Entropy succeeds quite well.
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Overall, to summarize:
Clarity of Power is typically associated with single target encounters, Auspicious Spirits is typically used when there are multiple targets to leverage Shadow Orb generation from, and Void Entropy doesn't see any use at all. At lower gear levels (where Crit and Haste levels are low, trinkets are non-optimal, and set bonuses are missing), using Clarity of Power for two target situations is viable.
There are also situations where randomness can heavily impact the performance of Auspicious Spirits. With RPPM trinkets that increase Crit and/or Haste, the generation of Shadowy Apparitions is heavily affected. The variance in Shadow Orb generation is so great that, while the average DPS in single target situations is lower than that of CoP, the standard deviation is much larger and leads to Auspicious Spirits being able to outperform CoP. As gear levels continue to rise, this will become more commonplace and less reliant on good trinket procs. The damage nerf to Devouring Plague in 6.2 may slow this slightly, since AS is more reliant on DP damage than any other Level 100 talent, but will be unable to stop AS's overtaking of CoP in the long term.
This is an example of a talent row that should give players a choice but results in noncompetitive options. Each choice has a significant impact on other talent rows and creates balancing headaches because they are trying to alter the spec too much. Effects from these choices are far reaching as playstyle changes drastically and further complicates and already complicated spec without simultaneously removing some complexity to make decisions more clear.
These thoughts are from a Heroic and Mythic PvE raiding perspective. Where appropriate, I try to tie back in how various aspects affect broader groups (such as Dungeon/Normal raiding or PvP as opposed to strictly Mythic raiding). There are many more specific topics, some even being favorite pet peeves within the Shadow community, which could be discussed at length but have been omitted or truncated. There are always topics available to dive in to at depth. For this post I've settled on Talents, Glyphs, Stats, Tier Set Bonuses, and the Hellfire Citadel Class Trinket. Many other topics are discussed inline (to varying extents) including resources, ability synergy, movement, baseline mechanics and their shortcomings/benefits, and others. This isn't to say that there aren't both good and bad things with these other topics that aren't worth discussing, just that they are not included on their own. One large piece that was purposefully limited and in many cases left out entirely are suggestions for how to adjust or fix things -- that's a whole other unwieldy and large topic that I'm positive will happen in the comments.
Breaking Down Talents and Glyphs
There are several talent rows that are quite well balanced and all have distinct homes in PvE. These talent rows are largely those that do not affect DPS rotations and instead provide some sort of secondary ability or effect that may help with fight mechanics or survivability. While this can't be the only goal behind a talent row, having meaningful and viable options available is key.
In general, talent rows that conform to a central theme help with immersion and fun. While this may not always be possible due to balancing or limits on mechanics (there are only so many ways you can achieve a similar affect before they all start being too similar), this is one area that seems to be lacking in some talent rows, specifically the DPS talent rows.
Level 15 Talents
This talent row is a prime example of a good mixture of choice and personal preference, all of which boil down to avoiding or recovering from damage. Spectral Guise has many great advanced uses to ease encounter mechanics; Desperate Prayer is an instant and large percentage heal for when you need a boost and have time to react to it; Angelic Bulwark gives the Priest a good passive shield (that is less effective, raw Health wise, than Desperate Prayer) that can prevent the Priest from taking fatal damage when you don't have enough time to react.
The talents are all themed around damage prevention/recovery in crucial/low health situations. Every talent has a logical use but doesn't limit you to always take one because it is 'better' than the others. This is an example of an excellent talent row.
Level 30 Talents
This talent row has two out of three options that are very good, and a third that doesn't have a place in PvE. Body and Soul piggybacks on Power Word: Shield to provide increased speed for a short period of time while simultaneously granting the damage absorption; Angelic Feather grants increased movement speed more often and for a longer duration; Phantasm prevents you from being slowed and removes any current slows on you.
The talents are all themed around two things: 1) movement speed, and to a lesser but still significant extend 2) reduction of damage by getting out of harm's way or mitigating it. This talent row is once again much about personal preference. The entire point of being able to move quickly is to get to a safe place/out of a dangerous spot. Body and Soul trades some movement for damage absorption (damage taken being what you're trying to avoid, ultimately) while Angelic Feathers lets you or others move for a longer distance to get out of harm's way (which, depending on the mechanic, may mean you take less damage overall than from a Power Word: Shield + movement via Body and Soul) and can be set up in advance, removing the rampup GCD when you really do need to move in a hurry and can predict it.
Phantasm is a bit out of place here. While it doesn't increase your movement speed above base (only back to), it also does not aid in reduction of damage. In every situation encountered during a raid where removing slow affects on you would be helpful to avoid taking damage, either a healer will dispel it (and is already looking for it to go out on a raid member) or it is the Shadow Priest's job to Mass Dispel, it can be trinketed (if you are a Human), the area you need to get out of isn't large enough / the effect is on enough of a delay that Body and Soul or Angelic Feathers will give you enough of a boost to move out of, or Dispersion or Glyph of Fade + Power Word: Shield is enough to absorb the damage you would take from failing to move.
For this talent row, two out of three are very good and gives a lot of choice. Phantasm, while fitting with the overall theme, does not offer anything extra. For a brief period of time, the 6.1 patch notes included a change to Phantasm that would make it viable in raids: add a damage reduction component as well. This is almost the same effect as Glyph of Fade, though, so likely only one could stay.
This is an example of a very good talent row.
Level 45 Talents
This talent row has a "default" talent pick and two others that rarely see use. Surge of Darkness grants procs based on Vampiric Touch and Devouring Plague ticks (via spent Shadow Orbs). Insanity grants increased single target damage and AoE damage when you spend Shadow Orbs. Mindbender is an amped up version of Shadowfiend.
The primary theme that is seen in two of the three talents in this row is Shadow Orbs granting new and interesting effects that affect how you do damage. Mindbender is the lone talent in this row that does not interact with Shadow Orbs at all. If you pick Mindbender, there is no managing of Shadow Orbs to be done (sans tier set bonuses altering playstyle).
For this tier, let's look at each of these talents in respect to the Level 100 talents, since those affect the playstyle of each (some more significantly than others):
Clarity of Power
Insanity is the default choice the majority of the time because it is not dependent on any randomness, and, it is controllable burst. This is key because of DoT Weaving, where you bank 5 Shadow Orbs, apply Shadow Word: Pain and Vampiric Touch (instead of using 2 GCDs of Mind Spike), then begin spending Shadow Orbs on Devouring Plague followed by Insanity twice back to back. There are many different versions of this depending on a player's Haste level and tier bonuses, but it is a slight DPS gain (~2%) in single target situations over casting Devouring Plague and using Insanity whenever it is available. This extra potential damage also comes at a large risk of DPS loss: if a DoT Weave rotation is botched by the player, either by removing DoTs from the target early with a Mind Spike, the target dying/becoming immune/out of range, or raid events interrupting it, the gain of DoTs vs. Mind Spikes for the two lead-in GCDs will be a damage loss. The core issue with DoT Weaving is that it goes against the rest of CoP from a stat priority point of view because less damage is affected by Mastery than with the intended CoP rotation due to the introduction of DoTs to a single target rotation. This leads to players building sets that maximize their DoT Weaving stats rather than focusing on Mastery; the end result is that most players who aren't playing at the top levels do less damage when they inevitably make a mistake in the play style and go back in to the Shadow Orb building phase that uses Mind Spike and Mind Blast exclusively.
Aside from DoT Weaving, Insanity has very good synergy with CoP's increased damage to Mind Sear for those burst AoE situations where there isn't time to apply DoTs. Insanity is also very strong when there are multiple targets that all need cleave damage with one being designated as a primary kill target, or, a new target appears and becomes priority where burst is needed.
Surge of Darkness is not as strong as Insanity in single target (DoT Weaving or otherwise) but grants the advantage of movement DPS if the need arises. This is good for less experienced players who may not be able to schedule their ability usage around predicted/known upcoming movement (where Insanity results in higher damage). Surge of Darkness's downfall is that it has some awkward interaction with CoP once you begin adding additional targets. Since Surge of Darkness has two ways to be proc'd, from Vampiric Touch or Devouring Plague ticks, to leverage additional targets to grant extra procs you must apply VT to some (or all) of these targets at the cost of: A) casting VT on all targets and using Mind Flay instead of hardcasted Mind Spikes on the primary target to avoid removing Vampiric Touch, and, not benefit from the bonus damage on Mind Spike from CoP, or B) only apply VT (and SWP) on the secondary targets and continue to use Mind Spike on the primary target as a filler. The design of CoP implies that DoTs should not be applied to your primary target, however, in many cases Priests achieve higher damage in this situation by applying DoTs to all targets instead of just secondary. This makes it easier to switch which target is considered your priority and, depending on how much movement is involved, results in higher DPS than having one target without any DoTs at all.
An additional concern exists for single target where SWP/VT will not be applied to the target. The proc rate of SoD from DP, when accounting for the damage of a SoD Mind Spike and DP ticks, is so low that it is better to remove DP with a hardcasted Mind Spike before it has been able to complete all of its ticks than it is to switch to Mind Flay while DP is on the target and wait for the ticks to complete. This is logically backward from how the talents are designed to interact with one another and is indicative of a damage balance and/or proc rate balancing problem. In 6.2 with the nerf to the damage that DP does overall (which affects how much damage it does per tick), this problem will become further entrenched. The damage, on average, of a SoD Mind Spike proc + Mind Flay as a filler + the tick damage from DP must be larger than the damage multiple (usually 2, sometimes 3) hardcasted Mind Spikes will do. Avoiding mechanics for a talent because playing the intended way is inferior to playing in what would usually be a haphazard and novice fashion is not fun.
Mindbender does not interact with CoP at all. It is merely a button that can be pushed three times more often. It's only advantages are that it gives the players an extra button available to press while on the move, and, there aren't any buff uptime or procs that can be wasted/overflowed if you have too many other things to do (such as refreshing DoTs on many targets).
Auspicious Spirits
Insanity is the default choice the majority of the time because of the constant (at high Crit levels) flow of Shadow Orbs to spend on Devouring Plague. Mind Flay is already the default filler spell for Auspicious Spirits so there is very little change in rotation when the buff is active more often thanks to having additional Devouring Plagues cast. When you have many targets where one is not the clearly defined kill target, there may not be an opportunity to cast Insanity as most of your time will be spent refreshing Shadow Word: Pain and casting Devouring Plague to avoid capping on Shadow Orbs. Searing Insanity is also able to see higher uptime as well, though how useful this is depends largely on fight conditions (target spread, how long they are alive, how often it occurs, etc.).
Surge of Darkness sees a substantial boost in situations where there are a few permanent targets to keep DoTs up on. If you have 3 or more targets, it may be difficult with high levels of Crit to spend your SoD procs due to needing to refresh DoTs on all available targets and cast Devouring Plague to avoid orb capping. As with CoP above, SoD excels when there are heavy movement phases to an encounter.
Mindbender does not interact with AS at all. It is merely a button that can be pushed three times more often. It's only advantages are that it gives the players an extra button available to press while on the move, and, there aren't any buff uptime or procs that can be wasted/overflowed if you have too many other things to do (such as refreshing DoTs on many targets).
Unlike CoP, AS is a spec where it is expected that you will not have many free GCDs to spend on SoD or casting Insanity, so having a "fire and forget" button is acceptable because there is no proc or buff to be wasted.
Void Entropy
Insanity is the default choice for a few key reasons. Due to Insanity being triggered off of any spent Shadow Orbs, not just Devouring Plague, you are able to apply VEnt to a target and still get a benefit from Insanity. While it should not occur more than a couple of times per encounter, VEnt has a slightly higher Insanity uptime per Shadow Orb spent ratio than CoP or AS because you are not GCD locked after casting VEnt (as it hast a cast time, whereas DP is instant) which results in an extra 2 ticks of Insanity. As more targets get added that you must maintain VEnt on, it becomes more and more difficult to save Shadow Orbs for burst -- you have a relatively small window where if you don't cast DP, VEnt will fall off of your target. This is also problematic for any required burst AoE from Searing Insanity.
Surge of Darkness's viability is diminished due to Shadow Orb spending competition between DP and VEnt. While a player may have increased proc chances from applying VT to these extra targets, there will be the lack of extra GCDs spent casting DP that AS has in the same situation. For the rampup period, when damage is already lower because VEnt is not out on all targets yet, will be even lower because of not having DP cast to grant extra SoD procs.
Mindbender does not interact with VEnt at all. It has an advantage over SoD in that the damage it does is not restricted by what abilities Shadow Orbs are spent with. As with AS and CoP, it gives the players an extra button available to press while on the move, and, there aren't any buff uptime or procs that can be wasted/overflowed if you have too many other things to do (such as refreshing DoTs on many targets or the inopportune casting of a DP to refresh VEnt).
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Overall, to summarize:
The L45 talents are a bit of a grab-bag of effects that have varying usefulness depending on what Level 100 talent you choose. Ultimately, Insanity is the go-to talent because of it's high uptime for CoP and AS (due to increased Shadow Orb generation) and for VEnt as well (because the spending of Shadow Orbs on VEnt instead of DP does not reduce the effectiveness). Surge of Darkness is well suited to multiple target situations for CoP and AS but not very much for VEnt. Mindbender is out of place in the scheme of things because it is not affected by neither Shadow Orbs nor any of the Level 100 talent, either directly or indirectly.
This is an example of a talent row that has balancing problems and a lack of choice induced by affects from another talent row.
Level 60 Talents
This talent row has a few niche choices that are heavily reliant on encounter conditions in PvE. Void Tendrils AoE roots, Psychic Scream AoE fears, and Dominate Mind charms.
This talent row is centered around CC. Having the choice between AoE roots and an AoE fear is nice and handy in a raid or dungeon environment, but Dominate Mind is just not fun (in PvE). Any time a player loses control of their character there is a high potential for unexpected, and potentially fatal, things to occur -- especially in a raid environment. This is amplified for a mechanic like Dominate Mind where you are unable to control your own actions at all due to literally controlling another character. With that being said, it is a staple and identifying ability in the Priest class's kit and very valuable in PvP. If there was a way that it could be more engaging without the large risk involved, perhaps as a taking control of mobs as pets in PvE so that the player can retain control over their own character (possibly silenced for the duration / breaking Dominate Mind on spell cast) that would remove or reduce the 'not fun' factor.
This is an example of a talent row that has a meaningful choice between two options and a third that is forced based on the encounter design.
Level 75 Talents
This talent row as a defacto pick and is missing a cohesive theme. Twist of Fate increases damage by boosting raw damage done; Power Infusion increases damage by letting you execute actions more quickly; Shadowy Insight increases damage by increasing resource generation in a random way.
The primary theme, though not one that is easily identified, is boosting damage. Twist of Fate accomplishes this by increasing the damage a rotation does passively; Shadowy Insight does this by making the overall rotation occur more frequently (at random); Power Infusion does this though a mix of both by speeding up the rotation and doing it passively (outside of pushing a button). Twist of Fate is the talent that wins out far more often than the others because it is passive, easily triggered, and sees significantly increased uptime whenever there are extra non-permanent targets.
As with the Level 45 talents, let's look at each of these talents in respect to the Level 100 talents, since those affect the playstyle of each (some more significantly than others):
Clarity of Power
Twist of Fate is the default choice. By procing at 35%, ToF is up for a large portion of the fight, most importantly including execute range. The execute phase for CoP is already exceptionally strong thanks to the increased damage to Shadow Word: Death (61% more powerful than a non CoP + non ToF cast). This isn't a necessarily a problem, but, with the high proc threshold it is very easy to toss a single SWP on a low add or off target that is not in execute range and have insanely high uptime on the 15% damage bonus for most of the fight.
Power Infusion has seen extremely limited usage with CoP due to it being a GCD locked playstyle -- there are no passive sources of damage (such as DoTs) to gain effectiveness from the Haste buff, it simply speeds up the rotation and makes it easier to be adversely affected by latency (where as with AS or VEnt, Mind Flay is more tolerant to latency at high Haste levels due to it being a channeled, 2 GCD ability). Additionally, due to Mind Blast having a reduced base CD, the haste bonus doesn't have as large of an effect (though it is proportionally the same as with AS or VEnt).
Shadowy Insight is almost never used with CoP. It has a very low proc rate (5%), meaning at a 1.25sec GCD if you never cast anything but Mind Blast->Mind Spike*3->Repeat, you would have roughly 1.8 procs per minute. Keeping this in mind, Mind Blast already has a reduced cooldown (3 free GCDs to use between casts), of which half of these procs (assuming every 4th GCD is Mind Blast, as it should be) from casting Mind Spike will occur when Mind Blast is already off cooldown; the other half of these procs will occur after the player has already begun casting their next ability and is GCD locked, or, will offset the entire rotation slightly and occurs when Mind Blast has less than 1 GCD (or slightly less) left on CD. The same situations occur when a player has SWP on secondary targets. In the best case, SWP will cause Shadowy Insight to proc while the player is GCD locked from casting Mind Blast or in the first GCD afterward. Otherwise, a proc will occur during the GCD before Mind Blast would come off cooldown anyway or in the GCD before where you are already mid-cast.
Auspicious Spirits
Twist of Fate is the default choice. As with CoP, ToF is up for a large percentage of the fight due to procing at 35%. The main driver behind Auspicious Spirits is leveraging extra targets to generate Shadowy Apparitions from SWP crits, meaning a player is already going to be applying SWP to anything that moves. If the encounter has adds of any sort that come and go periodically, Twist of Fate's uptime will be significantly higher than the baseline 35%.
Power Infusion is rarely used with AS presently and can not compare to ToF in single target situations; however, PI could be a viable DPS cooldown in a council fight (where ToF uptime can't be gamed) when Crit levels from gear get high enough to net consistent SA output. Looking to T18, there may be a possibility of using PI before the Legendary Ring is activated so that a Doppler Effect is in place with SAs flying towards, and striking, targets during the damage window to fuel as many Devouring Plagues as possible.
Shadowy Insight is rarely used with AS presently as the damage increase from ToF far outweighs the extra Mind Blasts and Shadow Orbs gained. At the levels where enough SWPs would be ticking on targets to get consistent procs, Mind Blast is already relegated to a filler ability in between keeping SWP up, casting DP before Shadow Orbs go to waste, using Shadowfiend/Mindbender, and casting your Level 90 talent.
Void Entropy
Twist of Fate is the default choice. The base uptime in single target results in roughly the same damage as Power Infusion (more on this in the next paragraph), but when adds start appearing it can lose its edge to Shadowy Insight depending on how long lived they are, or, how frequently they spawn.
Power Infusion's damage is slightly behind ToF in one target situations and much further behind in two or more target situations. This is because the first use of Power Infusion is either delayed until you have VEnt ticking on the target (or multiple targets), or, is used without any VEnts ticking at all.
Shadowy Insight is a viable option in certain situations. If there are multiple targets (2 or 3, more than this is difficult to maintain VEnt on), SI procs can help to get VEnt up on all targets more quickly, and, maintain it. Additionally, if you have many frequent waves of adds, you can also leverage Glyph of Mind Harvest on these new targets, with the increased cooldown on Mind Blast's effective impact reduced by SI procs, to maintain VEnt on many (3-4) targets.
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Overall, to summarize:
The Level 75 talents are largely one dimensional from a choice point of view. Twist of Fate, on some encounters in T17, can be abused so much that there is a 90% uptime. If this is the desired niche for ToF to fill (high up time when there are many adds coming and going throughout an encounter), then it should not also be the default choice in situations where there aren't targets frequently coming and going. Lowering the threshold % for the proc to occur will lower overall damage in single target but leave the potential uptime in encounters where there are many adds that will die at the presently seen levels.
With the removal of DoT snapshotting, Power Infusion's effectiveness in general is lower than it was during MoP. There are some potential use cases if just the right situation arises (gear + encounter), but these are few and far between. In general, with the Legendary Ring, pairing Power Infusion with Mindbender (from the Level 45 talents) could be quite strong in single target situations, regardless of which Level 100 talent is selected.
Shadowy Insight is not a viable option for CoP due to the small window in which to use it because of Mind Blast's reduced cooldown and being GCD locked. For AS and CoP, the proc rate is too low to be reliable -- during MoP, Shadow's Mastery increased the effective proc rate up to ~8% depending on gear, and, the Legendary Meta Gem plus DoT Snapshotting allowed for more frequent SWP ticks to occur. For VEnt, SI can be a bit of a saving grace in that it helps smooth out issues with having enough Shadow Orbs to maintain VEnt on many targets.
This is an example of a talent row that has virtually no choice due to balancing problems. Some talents work better for some Level 100 talents than others, but in general there is one talent that is head and shoulders above the others.
Level 90 Talents
This talent row has a defacto pick for single target and a defacto pick for multiple targets, but, has a cohesive theme. Halo does damage in a sphere around the caster based on how far away the targets are; Cascade does damage to targets based on the distance between source and end point; Divine Star does damage to targets in a straight line, out and back.
The theme of these talents is all about positioning and distance. For WoD, up until 6.1 Halo was the only choice available except in situations where there was a very high number of targets available, or, you would have a medium number of targets available more often than Halo would be available for use. Since 6.1, however, Cascade has become the overarching choice any time there are more than a single target due to Cascade bouncing to each target twice. This has left Halo as the default single target ability only because Cascade has nothing to bounce to.
Divine Star is out of the loop here. In MoP, it was very competitve due to the offheading it would provide (~15%-20% health to everyone, if stacked, every 15 seconds). With the removal of offhealing in WoD, Divine Star has lost its niche -- the damage was never good (even below Mind Flay from a DPET perspective in MoP). The new (or, rather, back to the original) use case for Divine Star is to use it when there are frequent periods of many adds stacked on top of each other very often. Out side of this very narrow window of use, Divine Star's damage is so poor in single target that it only beats Mind Flay in DPET until you get to high (100%) Mastery levels or medium (~67%) Mastery levels with the Mythic Class trinket. Compared to Cascade (where it is higher DPET than Mind Flay if you are not right on top of the target) for situations where targets are grouped, if there is even a single available target that is out of the group Cascade will win. There is no upside to Divine Star any more.
A general comment about the Level 90 talents: the distance component to each makes them unique abilities in the game, but is also what makes them incredibly frustrating to use. Halo is the worst offender due to it having a very specific maximum potential damage distance window. Without any built in UI to know how far away you are from your target, players are instead forced to use addons that can only make a best guess at the distance away they are when casting. This truly is a guess because there are limited amounts of information, including inconsistent information, to work with in an attempt to know your distance from the target. Some targets measure their hitbox from the middle of the target, others from the edge. Using class abilities that have fixed maximum use distances helps to get a general idea of the distance, however, attempting to peg 25 yards for a maximum Halo cast with this is unrealistic. The UI addition for Disc and Holy on Halo work well enough to give a rough, built-in idea of maximum distance but still suffers from the same issues as addons if it were converted to be used for DPS -- every target's hitbox is different. Compounding the positioning issue even more is trying to execute fight mechanics properly while the player is simultaneously trying to accurately position themselves, all without any accurate indication of if they are close to being in optimal positioning or not. Often times, encounter strategies make it impossible to be in position when using these talents (not just Halo -- all of them).
Divine Star suffers from this issue as well since it has a maximum 28 yard range (24 yards + 4 yard pulse) that, at the maximum distance, may register from addons as being able to do damage but in reality falls short of the target and does no damage at all, effectively wasting a GCD. Cascade is less affected by this as it reaches maximum power and has a 10 yard buffer window of usage, and, doesn't lose out on large amounts of damage for being off by a few yards, unlike Halo. Cascade is also unique in that, unlike Divine Star and Halo, it has to be cast on a target which allows for enforcement of using it only when you are in-range.
This is an example of a talent row that gives some choice to the player but selection is mostly dictated by encounter conditions. The abilities can be frustrating to use effectively as use of third party addons is mandatory and does not solve the source of the issue.
Level 100 Talents
This is a talent row that has a default pick based on how much gear (tier bonuses, trinkets, and item level included) a player has. Many, myself included, view this talent tier's theme as "sub specs" since each greatly affects Level 45 and Level 75 talent choices, changes stat and enchant priorities, and each vary the style of play much more than any other row of talents. An additional secondary theme can be seen as resource (Shadow Orbs) management. Clarity of Power has been described as the "DoTless" playstyle; Auspicious Spirits has passively increased orb generation that scales with targets; Void Entropy plays off of Shadow's heritage as a DoT class by granting an extra DoT.
Clarity of Power
This talent provides a faster paced playstyle where damage done by DoTs (SWP/VT) is limited, and, any preventing bonus damage on certain key abilities from occurring if Shadow Word: Pain or Vampiric Touch are present on the target. To achieve this, Mind Spike is introduced as the filler ability rather than Mind Flay for many reasons, most significant being that Mind Spike will remove DoTs from the target it is cast at, allowing any errant (or purposeful) application. Mind Sear also receives a damage increase on an individual basis based on if SWP/VT are on them. Shadow Word: Death also receives this substantial increase, making Shadow's execute even stronger than it normally is. The second component to CoP is the reduction of Mind Blast's base cooldown, which increases Shadow Orb generation, and making it instant cast, which gives CoP an ability that can be used while on the move (SWP fills this role for non-CoP playstyles).
Combining both of these effects helps to offset the passive damage lost from not having SWP/VT ticking on a target. This is also the source of some core problems with the talent: if there is a situation where you won't be using any of the abilities that receive a damage bonus, there is no penalty for using other abilities instead. This leads to strange situations like DoT Weaving (mentioned in the Insanity section of the Level 45 talents above) and CoP Lite, where the base Shadow playstyle remains identical to how it was in MoP but with a reduced cooldown and instant cast on Mind Blast. For multi target situations, CoP Lite is nearly always more damage overall than off-DoTing secondary targets and having a normal rotation on the primary target. Additionally, having DoTs ticking on all targets makes it easier to switch between targets when priority changes.
These alternative playstyles exist because ability options are not removed or restricted in a meaningful way. As it stands now, players need to know what the different CoP styles are and when to properly use each one to maximize benefit. Choosing the wrong style results in significantly reduced output. As mentioned above about Insanity/DoT Weaving, this is added complexity that does not result in a meaningful enough gain to be worthwile, with the combined downside that if you make any mistakes you end up far behind where you would have been.
The class trinket attempts to address this issue by doubling down on the damage that Mastery abilities do (plus Mind Sear). This may help to address some issues for single target encounters (specifically DoT Weaving, to an extent), however, the trinket will do little to change CoP Lite from being superior in multiple target situations since it increases the damage from both Mind Spike and Mind Flay evenly.
CoP is supposed to rely heavily on Mastery as its favored secondary stat, due to the majority of abilities that are intended to be used (Mind Blast, Mind Spike, and Mind Flay when DP is on a target) are directly increased by Mastery. Having DoT Weaving or CoP Lite as options reduces the percentage of your total damage which has Mastery as a contributing stat by a significant enough amount. This, coupled with Auspicious Spirits and Void Entropy not being reliant on Mastery for their damage, causes players to ignore Mastery in general in favor of other stats (specifically Crit because of AS) and further amplifies the effectiveness of CoP Lite or DoT Weaving rather than regular CoP.
Auspicious Spirits
The meta game for this talent is proper Shadow Orb use, specifically minimizing overflow when nearing cap. This is accomplished by providing a new source of Shadow Orbs that scales specifically with Haste, Crit, and the number of targets available. This is centered, specifically, around casting Shadow Word: Pain on as many hostile targets as possible to fish for Shadowy Apparition procs. At a medium numbers of targets (~3-4), Mind Flay is not ever channeled as there are no free GCDs while keeping SWP/VT on targets, Mind Blast cast on cooldown, spending Shadow Orbs on Devouring Plague before you are capped on orbs, and using your Level 90 talents and Shadowfiend. Above this number of targets (~5-7), Vampiric Touch becomes a filler when all Shadow Word: Pains are not at risk of falling off the target. At very high target counts (7+), Mind Blast may not even be cast as the player will receive more total damage (from Devouring Plague) from SWP than from casting Mind Blast.
This adds a gradual level of complexity and challenge to the talent where good decisions (when to drop certain abilities from the rotation) and high skill are rewarded. The largest drawback, however, is there is no built in notification system in place to track when you will get Shadow Orbs from Shadowy Apparitions spawning. Outside of using third party addons, players won't know how many or when they will receive extra resources to use. This can be confusing to players new to using this talent because of the unpredictable nature of their resources going from very few to enough to use other abilities in a very short period of time.
Void Entropy
This talent deviates from CoP and AS in theme by costing Shadow Orbs rather than increasing the generation of them. Spending Shadow Orbs to apply Void Entropy as a 1 minute long duration DoT means that you are unable to spend those Shadow Orbs on Devouring Plague causing the player to lose out on any specific secondary effects associated with Devouring Plague, such as fewer procs from Surge of Darkness (as mentioned above), less self healing, and reduced T17 4P uptime. The more times you have to reapply Void Entropy, whether it be because of having a new target or it falling from an existing target for a variety of reasons (phase transitions where the target is immune/target is out of range/players are stunned, too many targets to reliably keep VEnt up on) this same penalty occurs. Additionally, VEnt needs to be on a target for a certain minimum amount of time before it will do more damage than spending those same Shadow Orbs on a Devouring Plague. This varies based on the Spell Power coefficient on both Devouring Plague and Void Entropy, and, how much Haste the player has. In 6.2, with the nerf to Devouring Plague, the minimum amount of time at baseline (5%) Haste will be roughly 23 seconds.
Because Devouring Plague refreshes the duration on Void Entropy, though, it is fairly easy to maintain on a single target. Executing a normal rotation will keep VEnt ticking indefinitely. When a secondary target is introduced, it becomes more difficult to maintain VEnt on both targets especially at low Haste levels. With modest levels of Haste, assuming minimal delay in casting Mind Blast to generate Shadow Orbs, it is possible to maintain two VEnts. Above this, however, and without very specific encounter mechanics in conjunction with specific talents and/or glyph selections (see the Shadowy Insight discussion above on Level 75 talents), it is impossible to maintain three or more VEnts outside of execute range.
One key point to keep in mind is that, assuming you do not need to reapply VEnt more than once per target, the longer an encounter lasts for the more damage you get out of VEnt. This causes encounters where during progression VEnt may be an acceptable choice, once farm sets in and kill times drop substantially any damage advantage that VEnt may have had will disappear.
For less skilled players, where attacking more than a single target at a time is less likely to occur, Void Entropy has a strength in its simplicity: the rotation that players learn while leveling largely remains the same, the only difference is that at the beginning they press a different button a single time. In this aspect, Void Entropy succeeds quite well.
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Overall, to summarize:
Clarity of Power is typically associated with single target encounters, Auspicious Spirits is typically used when there are multiple targets to leverage Shadow Orb generation from, and Void Entropy doesn't see any use at all. At lower gear levels (where Crit and Haste levels are low, trinkets are non-optimal, and set bonuses are missing), using Clarity of Power for two target situations is viable.
There are also situations where randomness can heavily impact the performance of Auspicious Spirits. With RPPM trinkets that increase Crit and/or Haste, the generation of Shadowy Apparitions is heavily affected. The variance in Shadow Orb generation is so great that, while the average DPS in single target situations is lower than that of CoP, the standard deviation is much larger and leads to Auspicious Spirits being able to outperform CoP. As gear levels continue to rise, this will become more commonplace and less reliant on good trinket procs. The damage nerf to Devouring Plague in 6.2 may slow this slightly, since AS is more reliant on DP damage than any other Level 100 talent, but will be unable to stop AS's overtaking of CoP in the long term.
This is an example of a talent row that should give players a choice but results in noncompetitive options. Each choice has a significant impact on other talent rows and creates balancing headaches because they are trying to alter the spec too much. Effects from these choices are far reaching as playstyle changes drastically and further complicates and already complicated spec without simultaneously removing some complexity to make decisions more clear.