a Nightfall Shaman  

He looks to be random, but is not far SE of Obilisk of Souls and often is up standing still.

He's a real prick of a mob, which translates into another cheap and poorly AI programmed monster from SOE. In typical heroic fashion, you spank him because he's underleveled, and then he makes a glorious, mythical comeback at the last possible second and wipes you out with some mysterious and non-existent spell. If you're doing him for loot he's far from worth it, and as for Kragnok quest (giant heads), the regular giants are much better.