Luck Based
Ryu's face twisted with some disgust as he felt Pierthorn's power trying to return to him.
It was no surprise that Ryu had relied on the ability of his Perfect Blackbody Soul. There was truly nothing more effective against other souls than the devouring character of his own. However, the troublesome aspect of this was that he actually had to… devour.
It wasn't that Pierthorn disgusted him as a person, though sneak attacking a World Sea Realm expert when you already had a foot into the Sky God Realm was pretty despicable, the real reason his face twisted in disgust was because Pierthorn's memories had come with them, that and the fact he felt that this Mental Realm Master's Spiritual Qi was nowhere near pure enough.
Upon entering the Soul Quintessence Realm, Spiritual Qi was no longer referred to as such as was instead called Quintessence by some, Spirit by others, and some other yet still called it Spiritual Quintessence.