Why Does Nobody Remember Me in This World?

A time on earth there is a huge war between the five most dangerous rivals to supremacy on Earth. This war was won by humanity with the help of hero Sid. Then suddenly the story is rewritten after the eye of a boy Kai. He was the only witness that humanity defeated that war and now demons and monsters rule the world. This is because all the heroes from the world are absent and they face defeat. Kai was himself forbidden to all people in the world. He met with a magical girl named Rinne and with her help he escaped from the rewritten of his fate. He came out with a sword in the world of no hero and now he challenged all the powerful enemies on the earth.