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