The Nine Cloud Dream: Zero

Yang Soyu is better than you and he knows it. Even in his dreams, he is the chosen one. A martial arts grand master, Yukgwan, speaks to him in a dream and brings him into a parallel world with superhuman abilities and "errors," mutations that turn you into a monster. One of these errors is a former disciple of Yukgwan, now a dragon, and terrorizing the parallel world. Soyu and Kara, his anti-romantic partner, are chosen to hunt the dragon, but Soyu meets another who says he's the fake version of him...

(Source: Webtoon Entertainment)