Dark Vessel

From Diablo Wiki

Dark Vessels look a bit like Dark Cultists, but Vessels lean on tall crooks, and remain still in one place and ring the bells on top of their crooks, while chanting demonic verses until they transform, while Cultists are mage-like enemies who cast a variety of spells and summon demons.

Upon closer inspection, we can see that Dark Vessels have a spike on their backs. These spikes have been mentioned in the official Lore Entry for the Dark Cultists. They mutilate themselves by driving these spikes in their backs in their religious frenzy. Perhaps they show the initiation of a normal Cultist into a Dark Vessel. The lore entry has the spikes being hammered in the willing Cultists by a burly figure who may be a Dark Berserker.

Diablo III Bestiary
Bosses	Mistress of Pain • Siegebreaker Assault Beast • Thousand Pounder • Skeleton King
Demons	Activated Vessel • Dark Berserker • Dark Cultist • Dark Demon • Dark Vessel • Triune Cultist • Triune Leader • Triune Leader Blood Guard • Triune Leader Death Guard • Triune Vessel • The Fallen Ones
Undead	Crawling Torso • Ghoul • Grotesque • Skeletal Executioner • Skeletal Shieldman • Skeletal Summoner • Skeleton • Skeleton Archer • Unburied • Walking Corpse • Wraith • Wretched Dead • Zombie
Animals	Beast • Dune Thresher • Lamprey • Scavenger • Goatman • Goatman Shaman
Uncategorizable	Horned Ground Sloth • Ghostly Orb • Gnarled Walker • Pit Flayer Spawner