I'm Quitting Heroing - S01

When Demon Queen Echidna starts her invasion of the human realm, the people turn to the hero Leo Demonheart to protect them. Blessed with overwhelming power, Leo easily drives back Echidna's army and saves the world. But instead of gratitude or admiration, Leo gets disdain and scorn from his fellow humans, who fear his immense strength and believe it might one day bring about humanity's downfall. Banished from the kingdom he once called home, Leo wanders the land aimlessly until one day he hears rumors that Echidna is trying to rebuild her army. Hoping demonkind might accept him as an ally, he returns to the demon queen's castle and offers Echidna his help. Naturally, Echidna rejects him right away. Still, not all hope is lost, as Leo manages to convince Echidna's four generals to let him secretly work under their supervision. Taking on the identity of a masked dark knight named Onyx, Leo solves the demon army's problems one by one, gradually improving the quality of life around the castle. Above all else, though, Leo wants to accomplish one goal: to learn why Echidna started the war in the first place.