Naze Boku no Sekai wo Daremo Oboeteinai no ka?

Naze Boku no Sekai wo Daremo Oboeteinai no ka? (なぜ僕の世界を誰も覚えていないのか？) is a light novel series by Kei Sazane with illustrations by neco.

Plot
"Why does no one remembers the true world...!"

The era when the great race of the five tribes competing for hegemony on the ground ended with the victory of mankind led by hero Sid. However, that world was suddenly "overwritten" in front of a boy Kai. In the rewritten world, Kai saw the scene where humans lost to the five tribal wars because of the absence of hero Sid—here dragons and demons dominate the ground, and Kai is a forgotten existence from all human beings. However, after encountering the mysterious girl Rinne, Kai decides to break this rewritten destiny. In a world without heroes, he inherits the hero's sword and martial art and challenges the mighty enemy races who reign.