Shadow Fallen by Sherrilyn Kenyon
For centuries, Ariel has fought the forces of evil. Her task was to protect the souls of innocent mortals when they die. Captured by a powerful sorceress, she is transformed into a human who has no memory of her real life or calling.
And is plunked into the middle of the Norman invasion of England.
Cursed the moment he was born with a "demonic deformity," Valteri wants nothing of this earth except to depart it and will do his duty to his king until then. When a strange noblewoman is brought before him, Valteri realizes he has met her before…in his dreams. When others come for her, bringing with them preternatural predators, he is faced with a destiny he had no idea was waiting. One he wants no part of.
The truth is that Valteri isn’t just a knight of William the Conqueror. He is the son of one of the deadliest powers in existence, and if he doesn’t restore Ariel to her place, she is not the only one who will be in peril. The world itself hangs in the balance, and he is the necessary key to hold back the powers of evil. But only if he can find a way to work with the woman who stands for everything in the universe he hates.