Crimson Spell [SuBlime] Vol. 2 (manga)

Prince Vald is a proper, honorable swordsman by day, and a sensual, insatiable demon by night. The only one to know both faces of the cursed prince is his traveling companion, the sorcerer Halvir. During the day the handsome sorcerer keeps an aloof façade, even as he sates the demon’s desires nightly. However, Halvir finds he is starting to feel a deeper bond with the young prince. Meanwhile, enemies who want Vald for themselves are beginning to make their move.