Shoukan Yuusha, Maou ni Naru ~Maou (Ore) wa Yuusha (Ore) ni Taosareru Tame ni Maougun wo Tsukurimasu~

Kuruto was an ordinary high school student until he was summoned to another world as a Hero. He was told that defeating the Demon Lord would allow him to return home, so after overcoming countless trials, Kuruto finally destroys the Demon Lord and his army. With his mission complete, Kuruto steps into the magic circle that should return him to Earth. Instead, he arrives in the same fantasy world, ten years in the past. Even stranger, Kuruto has appeared at the very location where the Demon Lord’s castle will eventually stand. After defeating a monster girl who attacks him, he discovers that she is destined to become one of the Demon Lord’s closest retainers. Slowly, an impossible truth becomes clear. The Demon Lord Kuruto defeated was himself. If Kuruto wants to preserve the future in which his younger Hero self defeats the Demon Lord and earns the right to return home, then he has only one choice: become that Demon Lord. Now the former Hero must build the Demon Lord’s castle, assemble the Demon Army, gather the companions he remembers fighting against, and prepare everything so that ten years from now, the Hero he once was can arrive and kill him.

A time-loop fantasy in which the Hero already knows exactly how his battle with the Demon Lord ends, because he has experienced it from the other side.