Sin: Kurotoki-iro no Shoujo

One day, Kou meets a demon king..., a fatal encounter for him. Who can expect a demon king comes to your room? "I'm Cruel. You don't need to know who I am. You'll die soon." Kou can't understand what's going on.... Cruel's magic power divides into two and one goes to her and the other enters Kou's body. "You're now my follower!" On the next day, Cruel suddenly transfers to his class and his new life begins...
(Source: Himeya Shop)