After defeating the despicable Demon Queen, Leo Demonhart does not earn a hero's welcome, and those he fought to protect now regard him with suspicion and hostility. With nowhere else to go, Leo seeks employment with his former nemesis.

When Demon Queen Echidna begins her invasion of the human realm, the people turn to the hero Leo Demonheart to protect them. Blessed with insurmountable power, Leo easily repels Echidna's army, thereby saving the world. But instead of gratitude or admiration, Leo is met with disdain and scorn from his fellow humans, who fear his overwhelming strength and believe that it may eventually cause humanity's demise. Banished from the kingdom he once called home, Leo wanders aimlessly throughout the land until, one day, he hears rumors of Echidna's efforts to rebuild her army. Hoping that demonkind might accept him as an ally, he returns to the demon queen's castle and offers Echidna his help. Naturally, Echidna immediately rejects him. Still, not all hope is lost, as Leo manages to convince Echidna's four generals to let him secretly work under their supervision. Donning 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, however, Leo wants to accomplish one goal: to learn why Echidna started the war in the first place.—MAL Rewrite

After defeating the Demon King, former hero Leo Demonheart is learning that he was just a little too good at what he did, to the point that many now suspect that his real goal was to become the Demon King himself. Which, given that his name is Demonheart and he was created as part of a secret project, is perhaps not as far-fetched as it sounds, but now nobody wants to employ him, and a former hero has still got to eat. So, he applies for a job with the Demon King Echidna herself... but she doesn't want him either. Fortunately, some of her subordinates are more appreciative of his skills, and if he wears a disguise, he might just get away with becoming the new bad boy on the block after deciding I'M QUITTING HEROING.—Sentai Filmworks

After saving the world, the strongest hero Leo became someone who is not wanted in the peaceful human world. He was too strong. Exiled, he seeks a job at the Demon King's Army, which he had defeated and needs to be rebuilt. The Army had many problems: too much work, financial troubles, etc. Leo starts to make things better using his power. Leo encounters Echidna again, and asks her why she invaded the human world.—iQ