The Non-Existent Sky Demon Sword (無上天魔劍) also called as the Supreme Sky Demon Sword[1] is the Divine Sword Technique created by Cheon Ma after attaining enlightenment while studying spiritual energy and beings as well as Buddhism. It's a sword that can destroy everything and stop regeneration as well as the ability to cut the space[1][2] and was later passed to Cheon Yeo Woon when he attained Heavenly Master Level since it is a Divine Swordsmanship that could only be performed by ones who realized the mysteries of mother nature.
This is the completed technique for someone who mastered the use of Sky Demon Energy[3] that's why no one other than the First Cheon Ma and the Second Cheon Ma knew about[1]. Only when accompanied with a strong will and body will the Non-Existent Sky Demon Sword be able to destroy all things. The technique is surely powerful and frightening since it will surely lead for the ultimate death of an opponent.
In the case of the Heavenly Demon Patriarch, unlike the Second Cheon Ma, he used the completely Non-Existent Sky Demon Sword in a different way.