I'm Quitting Heroing

Demon Queen Echidna's invasion of the human realm prompts the people to turn to hero Leo Demonheart for protection. Leo's unmatched power drives back Echidna's army, saving humanity, but he's met with suspicion and disdain from his fellow humans, who fear his strength could be catastrophic for their kind. Leo is eventually banished from the kingdom and starts anew, eventually returning to Echidna's castle to offer his services as an ally. Despite being rebuffed by Echidna, Leo finds an opportunity to work with her generals, taking on the persona of a masked dark knight to improve the lives of those around the castle and uncover the reasons behind Echidna's initial invasion.