When the Demon Queen Echidna launches an assault on the human realm, the hero Leo Demonheart emerges as the savior the people rely on. Despite his unmatched power that easily thwarts Echidna's forces, Leo faces not admiration but rejection and fear from his kindred humans who dread the potential peril his strength represents. Cast out from his homeland, Leo roams the vast lands until whispers of Echidna's resurgence reach his ears. Desperate for acceptance, Leo offers his aid to the demon queen, only to be rebuffed. Yet, a glimmer of hope emerges as he persuades Echidna's four generals to let him assist covertly. Assuming the guise of the enigmatic Onyx, Leo endeavors to resolve the demon army's challenges, gradually uplifting the castle's surroundings. But amidst his deeds, Leo's ultimate quest remains unraveling the enigma behind Echidna's warlike actions.