Azaz'zel

Azaz'zel was once a demon who served under Belial, the Lord of Lies, but was also a spy for Dul'Mephistos. He was responsible for torturing Izual the angel that was captured after a failed assault on Demonforge and helped to turn him into a fallen angel. Azaz'zel was apart of many battles and sieges throughout the Eternal Conflict and was one of the oldest living demons in all of hell after the Great Evils themselves. He was supposedly in agreement with the Three Prime Evils in attempting to manipulate the Nephalem (humans) into siding with hell in the eternal conflict. Azaz'zel was finally slain by Belial during the Dark Exile as the Lord of Lies discovered where his true loyalties lay as he knew that if he did not, then he would have been slain by the spy and the Dark Exile would never have succeeded without the manipulative Belial at its head.

Trivia
In Islam, Azazel is known as the King of Devils, the enemy of Mankind and an ally of Satan himself.

He also is the archenemy to the Archangel Berathiel who is supposedly the lord of the Seraphim.