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

The protagonist, who had been transferred to another world, learned that defeating the Demon King would allow him to return to his original world. After overcoming numerous trials, he successfully subjugated the Demon King's army and annihilated the Demon King. Regretfully, he stepped through the magic circle for transfer back to his world, only to notice something strange about his surroundings. This was not his original real world, but apparently the same other world he had just left. And things were definitely amiss.

He defeated a monster girl who suddenly attacked him, and after questioning her, he discovered that this was the same other world, but ten years in the past. Furthermore, this was the exact spot where the Demon King's Castle had once stood. He also learned that the monster girl was a close retainer who had remained loyal to the Demon King until the very end.

Realizing that he cannot return to his original world unless he is defeated by his past self—the Hero—ten years from now, the protagonist resolves to become the Demon King in this dangerous realm, form the Demon King's army, construct a Demon King's Castle, and prepare to welcome his future self!