Slutty-Princess Diaries

Eight years after his mother's death, Distol learns that his sister, Princess Qoona, has been chosen as the holy maiden. The holy maiden sacrifices her own life to give power to the seal that binds the demon king. As Distol curses himself for not being able to protect the people he loves, a witch appears and makes him an offer that he can't refuse. She'll give him the power he's always wanted but only if he invades Bastarauge, kidnaps the princess Elena, and corrupts her from within.