Hwansan Jeonsaeng (2017)

An old man who never amounted to much as a martial artist finds himself in a position of leadership, because everyone with talent died in the great war of martial arts. At the end of his life he finds himself waking up in the body from back when he was a child. Equipped with knowledge on secret techniques only the leader of the sect had access to, he trains himself and sets things in motion to prevent the coming war from taking so many lives.

Source: www.anisearch.com/manga/21999