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

The hero, Fried, confronts the demon lord with his student, Lyle. Fried killed the demon lord after a ferocious battle. “I'm going to put an end to this war!” Just as he was about to finish off the demon lord, he is betrayed by his student who attacked from behind. Fried, who was supposed to be dead