From Tappytoons:
In this prequel to “Mookhyang: Dark Lady,” we meet Mookhyang in Murim, or the world of martial arts, as a young member of the Demonic Sect. After years of rigorous training, he rises through the ranks and joins an elite squad of first-rate assassins. Despite his impressive achievements, however, he finds himself hungry for more. His obsession with martial arts ultimately leads him down a different path. One that few have ever dared to take. He spends decades learning the way of the sword and cultivating his inner qi, or mana, in hopes of one day reaching a state no warrior of the Demonic Sect has ever reached before the State of Demon Transcender. Join Mookhyang as he fights powerful masters, takes down evil forces, and pushes himself to the limit and beyond.

Based on the first half of the novel.

Original Novel

Original Webtoon:
KakaoPage, Daum

Official Translations:
English, Japanese, S.Chinese, French, German, Thai