I mean, just take a look at the warlock spells.
1. Immolate, you are literally setting someone on fire. Do you know how horrific that would be? Your clothes melting to your skin, your skin melting and your eyeballs oozing under the heat?
2. Drain Life, could you imagine your health just deteriorating rapidly? Your body aging, your hair turning white and dropping out, your bones turning to dust, gross!
Imagine if WoW was built on CryENGINE 3. Could you imagine the horrifying things you would witness? A mages frost bolt, piercing through your chest, a shaman's lava burst melting your face off literally.
So what do you think it the most brutal spell/ability in the game? Take into account pain and suffering and the nature of the attack.