Crimson Spell is a yaoi manga by Yamane Ayano. The story is sort of a sexual Jekyll and Hyde. A young prince is cursed and enlists the help of a sorcerer to help him lift the curse. The curse turns him into a bloodthirsty (and horny) demon at night. The only way the sorcerer can subdue the demon is to have sex with him. The prince never has any memories of anything he does as the demon.
Crimson Spell is available in English in America from Kitty Media.