On a part of the map, near the Crimson Tree Forest, was an area called 'Demon Spirit Cavern'. It was one of the many mysterious zones in the Exotic Enclosure Region.
Tang Yin was almost 100% sure that the Demon Spirit Cavern contained Demon Spirits, hence its name. He had learnt of Demon Spirits from Tian Huang's jade token.
Demon Spirits were evil spirits that consumed each other and other stray souls to further their own strength and to obtain sentience.
They were usually stray souls that were forced to devour and consume each other to survive. Upon consuming enough souls and spiritual entities, these stray souls would become Demon Spirits.
These Demon Spirits would then obtain an innate ability, Soul Engulf. It was similar to Tang Yin's Soul Refinement Technique, however, the drawback was that Soul Engulf could only be used on other Demon Spirits.