Azael the Ensnared is a fallen Aasimon who became the lord of the Savage Searing, which is the Layer of the+Abyss now claimed by Yeenoghu, catalogued as the 422nd. He had been a "hero" in the war against the obyriths, and became the demon prince of rebellion and war, with an extensive cult on the Prime Material Plane. In ancient days, he was bound to an enormous plaza on his layer by magical chains, yet still none could overthrow him. That is, none until the god-killer Ma+Yuan invaded. Only an oily black stain remains of Azael, a black wound in the Abyss.
During Azael's reign, the 422nd layer of the Abyss was a sprawling desert of shifting dunes and doppled cities. When Yeenoghu conquered it after Azael's death, he transformed much of the layer into savannas and woods.
Source: Fiendish Codex I, page 147