When Nagyunn’s twin brother, Najin, is murdered before his very eyes, he vows to avenge his brother’s death by assuming Najin’s identity. But although the two may look alike, when it comes to fighting, Nagyunn lacks the talent and skill his brother possessed. In order to successfully execute his plot for revenge, Nagyunn resolves to train himself to become the prodigious knight-in-training his brother once was. Will Nagyunn be able to keep his identity a secret before others begin to catch on to his act?

There is a work that have a universe like this series:

Epic of Gilgamesh: Same universe