The soul could possess another body, but ghost cultivators could not.
Ghost cultivators were born from the Yin Essence between heaven and earth. They transformed into wandering souls, awakened their intelligence, and began cultivating. Alternatively, they were formed from the remnants of soul power left behind after the souls of many martial artists on a battlefield entered reincarnation, gathering together to give birth to wandering souls.
A martial artist's soul could not transform into a ghost cultivator. After a martial artist died, their soul could leave the body and exist briefly for a short period of time. If they tried to transform into a ghost cultivator during that period, they would be violating the rules of heaven and earth, attempting to escape the shackles of the six paths of reincarnation. They would inevitably be blasted apart by the laws of Heaven, their souls scattered and spirits dispersed.
Therefore, many extremely powerful martial artists in the Spirit World, once they felt that breaking through their cultivation realm was hopeless and sensed that their lifespans were nearing their ends, would search everywhere for secret arts and treasures if they intended to become ghost cultivators. Before their lives ended, they would free themselves from their physical bodies and transform into ghost cultivators. This process was known in the Spirit World as "Corporeal Dissolution."
As Raven had said, if the Nine Yin Ghost Physique appeared in a martial artist, it was called the Nine Yin Ghost Physique. But if it appeared in a ghost cultivator, and that ghost cultivator was a woman, then it was the Nine Yin Spirit Nurturing Body.
After carefully sorting through everything Raven had said in his endless stream of words, Lin Bai roughly understood the situation... The Rakshasa Ghost Emperor had discovered that Yue Shuang possessed the Nine Yin