The black mist originated from the main body of the remnant will.
Since Li Yunmu had supplemented the flaw in his flux points after descending, he had almost an identical constitution to a native of the Origin World. Because of that, if he returned to his Earth, it was highly possible that he would become the most ideal target for the remnant will's main body to seize.
Why would he be the most ideal target? Because the main body of the remnant will was from the Origin World.
Slowly, Li Yunmu understood everything. He would have never expected that the black mist around his homeland was surprisingly because of an ancient god's deity soul which had existed for more than ten thousand years, according to the remnant will.
As a result, not only he, but other descenders like Hai Yue, Xiye Hanbei, or Man Tian also couldn't return to Earth.