Naze Boku no Sekai wo Daremo Oboeteinai no ka?  

Why Does Nobody Remember Me in This World?, なぜ僕の世界を誰も覚えていないのか？, Why nobody remembers my world?, โลกคู่ขนานกับตำนานวีรบุรุษที่ถูกลืม, NazeBoku

"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 humanitys 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 humanitys defeat in the war as a result of Sids 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.(Source: MAL News)