Matanzas the Loathsome
From Diablo Wiki
Matanzas the Loathsome, "Wielder of Elements", is a Superunique Exorcist that can spawn in the Westmarch Commons in Act V. It uses the Molten, Poison Enchanted, and Frozen Boss modifiers. Its appearance is identical to Yergacheph. Matanzas the Loathsome is one of the monsters to be killed to get the Special Assignment and Irreplaceable You Achievements.