Baek Suryong was once a part of the Blood Demon Cult martial arts sect, until he was betrayed and killed by them. He wakes up in a new, sickly body some years later. Once an abusive instructor for young Blood Cult members, after reincarnating he decides to heal himself and become recognized as the number one instructor at a certain academy. In the process, he uncovers a greater conspiracy. This is the basic premise, but it does not illustrate how much of a hooligan Baek is. Our main character turns over a new leaf but does not become a generic paragon of justice and even gets on many peoples' nerves. His own philosophy clashes with that of other martial artists of the "righteous" path, he realizes that there are some things he is unsure about because of his upbringing in the Blood Cult, the greater conspiracy means that there are few people he can trust, and his body's disposition means that there are some martial arts techniques that he is physically incapable of using. In other words, he's a bit OP but the conflicts in the story keep him from becoming a Gary Stu. 10/10
A historic martial arts series adapted from a Korean webnovel. If you like this series, I would reccomend "Return of the Blossoming Blade."