Naze Boku no Sekai wo Daremo Oboeteinai no ka?

There was a war on the earth for the supremacy of the earth and in there five rivers face each other. Humand won that with the help of Hero Sid. Now humans rule over the earth and eliminate all other things. Then the fate of the world is rewritten in front of a boy, Kai. Everyone forgets him and he is the only witness that humanity lost the war and now demons rule over the world. With the help of a magical girl, he ran away from his fate, and now he wanted to save Earth. He with his sword challenged all the powerful dominate in the land.