Saikyou no Maou ni Kitaerareta Yuusha Isekai

Without possessing the unique skill blessed by the goddess, a boy named Kujou Kyouya failed to save the world. He was ridiculed as a “failed hero” at the interdimensional-level academy where returned heroes who had saved many worlds gathered. However, he became the strongest and most unique hero, having trained for 30,000 years in another world with the most evil demon king, Felice, who has destroyed 3,000 worlds! Now Kyouya’s unique skill is so powerful that he cannot even be defeated by the strongest heroes! The legend of a boy trained by the most evil demon king, begins! (Source: Mangamonster.net)