Ran into him in Inferno with a friend, changed from wraith to Flesh of Nar Gulle, those lovely poison corpse cloud critters that come from decaying body piles....ultimately turned back into wraith and is unkillable.
I'm a monk, Sweeping wind seemed to damage him, but melee hitting him did not refresh my sweeping wind though it did give me spirit AND life on hit. Brought him down to a totally empty health bar, kept at it for 5 mins....no change. I have dmg numbers turned on, he shows no damage taken numbers from any of my abilities (even when I can obviously see his HP's decreasing).
I intentionally died, afk'd in town a bit to relax, came back, he was at 50% health...odd, any other elite I run into full heals unless I drag them to my spawn point and am under a 15 second death timer. Anyway, I wore him down with wind some more (takes forever since crits do not improve it, and nothing extends it)....same story, empty health bar and not dying.
Got here, saw this post from BigRay regarding a bunch of rotting bodies, went to find some left in the dungeon, trashed more of those corpse poison cloud leapers, diligently brought him through their poison clouds....nothing happened, tried the same with some bloated corpses for good measure....no dice. If there's some secret way to kill him after he bugs out and won't take damage, I can't find it.
Anyway, seeing as how this post started forever ago, is filled with ppl confirming it's bugged, and blizzard has no posts regarding it (shocker). I'm assuming I'm just wasting even more time typing this...however in hopes of saving someone elses' evening (my friend is in the background beating on it while I type this....another 10-15 minutes?). If you run into this lovely critter, just pass him up, it's an even bigger waste of time than farming for drops that barely exist.
On a positive note...I cleared the rest of the sewers with him chasing me, 2 elite packs, bunch of trash....as far as elites go he's a joke other than the whole godmode thing and the annoying vortex from time to time.
EDIT: Ran into him and trashed him in short order today, perhaps only bugged if not killed in a timely fashion....did smash him before worrying about the corpses this time, standard purple mob trash loot.
Edited by Silster#1641 on 7/31/2012 8:57 PM PDT