A zombie's soul body was not actually complete.
After a person died, if they did not become a ghost, their three souls would dissipate first. Some time later, their seven po would also return to heaven and earth.
The three souls governed yang, while the seven po governed yin.
Once the three souls dispersed, the seven po lost the nourishment and sustenance of yang energy, along with the force that held them together. The flesh would gradually crumble and return to the earth.
If the corpse happened to be buried in yin ground, the yin energy would keep the seven po together, producing a stiff corpse that would not decay.
If, through a chance coincidence, that corpse also acquired the souls it lacked, it would break out of the earth and become a "wild" zombie.
To control such a "wild" zombie, a corpse driver had to pay a far greater price than for one produced through normal corpse refining.
The Zhong Clan's Secret Manual of Corpse Driving had mentioned one particular situation.
Once a zombie that had formed naturally through chance circumstances grew to a certain point, a special opportunity could cause it to undergo "spirit awakening."
At that point, it would be different from ordinary zombies.
It could even be called a new species.
A zombie that underwent spirit awakening would gain humanlike intelligence, but none of the memories from its former life. It was more like a "newborn."
Ordinary newborns came from their mothers' wombs, while these "newborns" were nurtured by heaven and earth.
For a corpse driver, trying to control a zombie in that state was no different from trying to control a living person directly.
Thus, the "po" were the key to a corpse becoming a zombie, while the "souls" were the key to controlling it.