Five hundred years ago, Great Demon King Belphegor nearly conquered humanity before a Hero defeated him. Now a cult has summoned his soul into the sacrificed corpse of a young woman, leaving him trapped in a frail undead body sustained by Mana Stones. To survive, Bel hides his identity as a witch, chooses the earnest hunter Lucius as the next Holy Sword Hero, and becomes his mentor. With Demon Kings returning and his old followers moving in the shadows, saving the world may be easier than keeping his past secret.