Esiel, Guardian of Malthael is a Super Unique Anarch found in Pandemonium Fortress Level 2 in Diablo III. He is one of the three (the others being Xoren and Zalud) Guardians who can spawn in the area.
Esiel, unlike other Guardians, can spawn anywhere in the Fortress. Killing him is not required for the quest.
In combat, Esiel has Desecrator, Fast and Frozen affixes.