In a certain world, there was someone who excelled in magic combat, people called him "Sage".
Seeking the strongest combat art, he devoted himself to research every magic and combat arts in existence.
And the conclusion he arrived at was very cruel: "My body is not fit for magic combat".
Yet he didn't give up.
He sealed his own soul with a magic art, to be reborn in a far-off future.
And in the completely-changed world he saw in his new life, the magic theory was of unbelievably low level.