According to Elder Shu, when the human and the Demon Emperor plunged into the Human Realm, it was highly likely that their Void Realm merged with the Human Realm as well. Otherwise, where would the energy fueling the Human Realm come from? Even immortal cultivation must obey the basic laws of conservation of energy.
Even with his cheating abilities, energy could not be created out of thin air; it had to be transferred from somewhere resembling a higher-dimensional space.
Back then, Yu Xian had inadvertently discovered a way to reach the Void Realm through the True Interpretation of Laws and made it his own.
Chapter 365