Lv1 Maou to One Room Yuusha

Ten years ago, the hero Max defeated the Demon Lord. In order to regain his power, the Demon Lord went into a deep slumber...but when he awakens, his body is only a chibi version of its fearsome form. Now curious to see what his enemy is up to, the Demon Lord visits Max, only to discover the mighty hero living in a dirty one-room apartment. The Demon Lord decides to move in with Max and help his old enemy become a formidable opponent once more!

(Source: Seven Seas Entertainment)