Since my undead servant rarely dies, I haven't noticed this until recently. BUT, when he did eventually die in front of my eyes, I noticed he didn't perform a death animation or leave a (less mobile) corpse. Kind of a small thing, but since Abominations are a reused model anyway, why do they fade when killed? There are all the necessary animations already in game! Is this a bug, or did Blizzard forget to add them?