Evil spirits are not common within ancient battlefields, even those on a grand scale.
In ancient battlefields, evil spirits usually exist in a sealed state, like Liu Xi, who was bound by Soul Binding Tungsten Crystals.
Soul Binding Tungsten Crystals attached to the surface provide a relatively stable living condition for evil spirits.
Otherwise, during the long course of time, the spirit would have turned into dust before they fully transformed into an evil spirit.
They would become a wisp of abhorred filth.
There are two possibilities for an evil spirit to break free from the Soul Binding Tungsten Crystal.
One is the natural weathering of Soul Binding Tungsten Crystal under the erosion of years.
This possibility is minute and requires extremely specific and harsh conditions.
These specific harsh conditions, however, are hard to come by.
Once such conditions are met, there certainly would give rise to more than just a General-level Evil Spirit.