Dark angel

A dark angel is an angel who has turned away from the light, and instead embraced darker powers. Unlike fallen angels, they are generally assumed to be malevolent rather than simply rebellious. Depending on the story series, they may/may not be the same as demons (likely not, given that the distinction is being made at all).

Aeon of the Champion (AoC Universe)
The Dark Angel is one of the titles of Sargamon, Dark Lord of the Marching Horde and Great Enemy of all Life. It could also be used in reference for his short stint as a neutral fallen angel (Lord of the Outlands).