Sealing the whole cave shut, he entered seclusion.
Before, he had left his trace of aura along with an alchemy formation here, meaning that he was here so that others wouldn't enter this mountain and disrupt his seclusion, and even if they entered he would immediately know.
The cave had small holes in some places so it could ventilate the air properly.
...
Black smoke came out from the eyes and mouths of the three skulls hovering above his head.
The smoke seemed to be full of death and sickness, just a single touch of this aura and a normal human would be eroded by it, experiencing immense pain before dying.
For some special cases, the person would enter a symbiotic relationship with the black smoke before degrading into an undead.
This was the essence of the three undeads, and their whole lifetime of cultivation was stored in this black smoke.
As the black smoke come out, it would enter his seven orifices and disappear.
