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! 
 
(Source: Crunchyroll)

"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.

"Why am I forgotten from the whole world?" 
 
War between the five races on earth - the humans, the "Demonic Race", the "Wild God Race", the "Spirit Race" and the "Phantom Beast Race"- has ended in the victory of humans. It is said that "Prophet Sid" had sealed the other four races in the black pyramid now called "Crypts". Kai's job is to watch the crypts and make sure that there is no change. He is also training to save humans when the other races break the seal and come back to this world. 
 
The crypts looked quiet today...but after Kai felt the world bending in front of his eye...everything changed. "World Rebirth" was activated...!! Now Kai is in a totally different world where no one knows him. But he remembers everyone!
 
(Source: BookWalker)

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