Manga Tsuyokute New Saga

The terrifying demon king has gathered a huge army of minions to deal with humanity once and for all. However, there is hope, Cale and his allies have attacked the castle of a terrifying monster to destroy it. They managed to do it, but the price was terrible. All close boys, all allies, all those who were dear to him, perished during this horrific war. And he himself dies from a serious injury. However, just before his death, he manages to find a strange gem, which the Demon King protected with all his might. After he took it in his hand, he realized that he was safe and sound in the distant past, when the war had not yet begun, everyone was alive, and he was young and full of strength. Now he is faced with the responsible task of saving all loved ones and humanity from a terrifying future.