Manadeshi ni Uragirarete Shinda Ossan Yuusha, Shijou Saikyou no Maou toshite Ikikaeru

Fried, the hero, faces off against the demon lord alongside his student, Lyle. After a brutal battle, Fried defeats the demon lord, declaring an end to the war. However, Lyle betrays him, striking from behind. Fried, presumed dead, awakens beside a demon girl who addresses him as the new demon lord. He discovers he has been reborn as the ruler of demons and must now defend them while confronting those who once fought alongside him.