I'm Quitting Heroing

Leo is a hero who once saved the world from a demon army's invasion. However, he was feared and ostracized by people afterward, and ended up with no job. Feeling that his skills would be better appreciated elsewhere, he joins the demon lord's army that he once fought. Now in a new work environment, Leo strives to use his skills and revive the demon army that is on the brink of collapse. <em class=de-emphasized>(from manga)</em>

Synopsis from Anime News Network