The Sword Art of the Demon God (魔神劍功) is the completed version of the Sword Art of the Sword God that Ancestor Cheon Ma left unfinished. The sword technique created by Cheon Ma after fighting Blade God at the Sword Creek but was incomplete due to it being "impossible" because of physical limitations since Extreme Art of the Blade God requires specific training of muscles.
After falling to the cliff and bottom of the sword creek due to the explosion, Cheon Yeo Woon saw the traces of battle left at the area. He then saw an unfinished traces where the techniques are being combined. Cheon Yeo Woon scanned the traces, gain insight, and he successfully combined the Extreme Art of the Blade God and Sword Force of the Heavenly Demon and named it as the Sword Art of the Demon God.
Although Cheon Yeo Woon presumed that Cheon Ma couldn't complete the technique due to the physical limitations of the Extreme Art Of The Blade God, Cheon Ma had actually mastered the Extreme Art Of The Blade God and was capable of combining it with his own martial arts. However, he chose not to complete it because the resulting sword technique would have been too difficult for anyone to practice except himself.