Denizens of the Spirit Realm. They are typically devided into the undead or celestial category, depending on the nature of their formation.
Undead spirits are formed from living beings. Typically a traumatic death is the origin of an undead spirit, and will inform its nature. Records exist of undead spirits exalting into celestial spirits.
Celestial spirits are spontaneously formed. They often embody an object or a concept, such as a spirit of a river or forest, or a guardian spirit of a village.