Yi Yun knew that his final strike would not be able to penetrate the Black-armored Demon God's skill, hence he had added the 'evil energy' that the Purple Crystal controlled on the sword.
This evil energy had entered the Black-armored Demon God's soul sea at the moment Yi Yun pricked him!
Ever since Yi Yun entered the Heavenly Dao Union, and took control over Shentu Nantian's corpse, the evil energy had once again been in Yi Yun's control.
The evil energy was adept at devouring and possessing souls. Back then, it had devoured the masked man, allowing Yi Yun to make the masked man his puppet and read his memories, eventually destroying Blood Moon's 'Blackstone Trials' plans.
Back then, the evil energy had grown a lot. After that, in Blood Moon's Soul Tomb, Yi Yun had used the evil energy to devour all the sleeping Yin specters!
After devouring so many Yin specters, the evil energy was greatly enhanced.