boys-loveimprintpublishes the manga in English, and it describes the story:
Upon wielding an ancient sword that has been passed down through his royal family for generations, Prince Vald is struck by a curse that turns him into a demon! In search of guidance, Vald appeals to the powerful sorcerer Halvir for help, and the two set out on a journey to break the curse. However, there is one thing the handsome sorcerer is keeping secret from the young prince the raging demon that Vald turns into every night can only be calmed when Halvir satisfies his lust! And so begins their epic journey in search of clues to break the young prince s curse.