Yuusha, Yamemasu

When Demon Queen Echidna began her invasion of the human realm, people called on the hero Leo Demonheart to protect them. Blessed with incredible power, Leo easily repelled Echidna's forces and saved the world. But instead of appreciation, Leo was met with distrust and fear due to his overwhelming strength. As a result, he was banished from the kingdom he once called home.

Leo heard rumors of Echidna rebuilding her army and decided to offer his help, hoping he would be accepted by demonkind. He returned to the demon queen's castle, but was rejected by Echidna. However, he managed to convince her four generals to let him work secretly under their supervision. Taking on the identity of a masked dark knight named Onyx, Leo used his power to solve the demon army's problems and improve the quality of life in the castle.

Above all else, Leo wanted to find out why Echidna started the war in the first place. He hoped that by working with the demon army, he would be able to uncover the truth.