When Demon Queen Echidna launches her assault on the mortal realm, citizens appeal to the champion Leo Demonheart for defense. Endowed with boundless might, Leo swiftly drives back Echidna's forces, thereby safeguarding the planet. Yet instead of thanks or praise, Leo faces contempt and derision from his compatriots, who dread his immense power and suspect it could ultimately bring about humanity's downfall.
Exiled from the land he once inhabited, where he had defended the realm with unwavering resolve yet his peers turned hostile after his triumphs, Leo roams without purpose until he learns of Echidna's plot to rearm. Desiring acceptance from the demonic realm, he ventures back to the queen's stronghold and pledges his aid.
Echidna promptly rebuffs him. Nevertheless, hope persists as Leo persuades the queen's four lieutenants to allow covert service under their watch. Assuming the guise of a masked knight called Onyx, Leo tackles the army's issues one after another—gradually enhancing conditions around the fortress. Above all, Leo seeks to uncover why Echidna ignited the conflict.
He longs to prove that strength can be wielded for peace, not destruction. And to restore trust among the populace.
