Shinma Gyeongcheongi

In an ancient world where martial artists reign supreme, Un-Seong can only watch as his master is brutally beaten to death after a false accusation of practicing forbidden demonic arts. Even after a valiant fight, he fails to kill those responsible and faces his own end. But fate has other plans… Instead of taking his last breath, he awakens as a child, training to be an elite soldier of a demonic sect. Now Un-Seong must embrace the demonic arts and harness power in his ultimate quest for revenge.
(Source: Tapas)
Note: Includes six extra chapters.