Hazza'rah (WoW)  

He's one of the 4 possible bosses that spawn at Edge of Madness when you use the Alchemy potion on the brazier. For tactics, just have people with instant damage spells drop the illusions the second they appear (theres a smokey effect just before, so that you can prepare). Shamans or Mages are pretty good for this, since they dont move very quick, and have ~500hp, but they'll hit for 5k+, so getting rid of them is a priority. It also helps to "hide" the main tank in the middle of the raid, so one of the illusions doesn't drop him.

Spells:

He deals shadow and nature damage

Has a mana burning chain attack

Casts AoE sleep (affects everyone) during which time he attacks someone

Summons 3 Nightmare Illusions continuosly during the fight