Cornelius could not see the ghost, but he could sense its energy.
Ghosts were usually not prominent, especially before powerful beings. Most were just souls that drifted in confusion for a while after death before eventually disappearing from the world.
Most people, including supernatural creatures, could not sense their presence. Their energy was minuscule and not harmful. However, malicious spirits had an obvious oppressive aura of death.
But malicious spirits were not common.
The spirits would either be filled with deep grudges from inhumane deaths. These spirits were dealt with by Yin-Yang masters who could force them to dissipate or reincarnate. These were secrets of the cultivation world.
In other cases, the malicious spirits were enslaved ghosts captured by cultivators to increase their power. The technique was considered evil because ghosts were, after all, human souls that were denied peace in death.
