When chaos first separated, rules were established across the three realms of heaven, earth, and humanity. In the Celestial Realm, with four gates, only one could be opened at a time. Whichever gate was opened, the guardian would become the acting Heavenly King, and the other gates would not appear.
This was a rule, a rule that no one could defy.
To this day, there has never been an occurrence of two gates opening at the same time, and nobody knows what problems would arise if it truly happened. But it is certain... that two Emperors would definitely not be found smoking, drinking, and watching movies together.
That's why Taiyi was so panicked, and the civil war within the Demon Gate of the thirty-three heavens might very well affect the Human Gate.
"As for the realms of heaven, earth, and humanity, the Heavenly Gate is actually the Demon Gate; what about the Earth Gate?"
