Level 1 Demon Lord and One Room Hero

Max, the hero, defeated the Demon Lord Maou in a climactic battle, saving humanity. Ten years later, Maou returned but due to incomplete resurrection, he's now a child. Maou moves in with Max, hoping to help him regain his former strength and become a worthy adversary. However, as he learns more about Max's life during his decade-long absence, he discovers that there's more to Max's story than he initially thought.