The Tianhe light is the absolute nemesis of the Dead Spirits!
In fact, the Path of Wood of the Old Azure Dragon is vibrant with life, while the Shadow Dead Spirits represent absolute death. These two paths are like water and fire, mutually incompatible yet restraining each other.
It's not that the path of the Old Azure Dragon restrains the Shadow Dead Spirits, but rather that he is too powerful. It's like what's a fire against a deluge? Similarly, a cup of water cannot put out a raging wildfire!
But the Tianhe light is different, it is the absolute restraint!
Back when Zhou Heng was merely at the Comet Realm, his Tianhe light could literally burn and meld the body of Black Cave level Dead Spirits, although the sheer strength of the Black Cave Realm allowed them to escape with their lives!
However, now Zhou Heng himself is at the Black Cave Realm, when the Tianhe light appears, even Black Cave Realm Dead Spirits are doomed to instant death!
This is true restraint!
