Saikyou no Maou ni Kitaerareta Yuusha, Isekai Kikansha-tachi no Gakuen de Musou Suru

Kujou Kyouya initially failed to save the world, lacking the divine unique skill typically granted by the goddess. Because of this shortcoming, he is scorned as a "failed hero" within an interdimensional-level academy—a prestigious institution where elite heroes who successfully saved various worlds now gather. Despite this harsh reputation, Kyouya harbors a staggering secret. He spent 30,000 grueling years in another realm training under Felice, the most malevolent demon king responsible for the destruction of 3,000 worlds. This ancient and brutal tutelage transformed Kyouya into the strongest and most unique entity possible. Now possessing a power so overwhelming that even the academy's most formidable heroes stand no chance against him, Kyouya's unstoppable legend finally begins.