After the great war ended with humanity's victory, the world suddenly gets "overwritten" right before the eyes of a boy named Kai.

"Why does no one remember the true world......!" In a time when the great war between five rival races for supremacy on Earth ended with humanity's victory led by the hero Sid, the world suddenly gets "overwritten" right before the eyes of a boy named Kai. In this rewritten world, Kai witnesses humanity's defeat in the war as a result of Sid's absence-where dragons and demons now rule the land, and Kai himself has become a forgotten existence to all humans. However, after encountering the mysterious girl Linne, Kai resolves to break free from this rewritten fate. In a world without a hero, he inherits the sword and martial skills of the hero (Sid) and challenges the powerful enemy races that dominate the land.—MAL News

In a world where humans triumphed in a great war, Kai guards sealed crypts containing their enemies. But when "World Rebirth" overwrites history, he's thrust into an alternate reality where humanity lost the war and he's been forgotten by everyone he knows. Now, this lost hero must rise to restore the world's balance!.—Crunchyroll