The Spirits represented supernatural entities in the belief system of the Nightsisters from the planet Dathomir. Both the Jedi and Sith generally considered these "Spirits" to be rooted in an ancient and shamanistic understanding of the Force. The Nightsisters held the conviction that the Spirits favored Dathomir with blessings, enabling the practice of magicks through the channeling of their energy.
The Nightsisters believed in a separate reality, a plane parallel to the physical one, populated by spirits. This spiritual domain was thought to be ruled in equilibrium by the Twin Deities, specifically the Winged Goddess and the Fanged God, who embodied the Light and dark side of the Force.
In her instructive work Wild Power, Clan Mother Talzin suggested that each animal species had a singular representative spirit in the spirit realm, which she termed the "ur-spirit." These ur-spirits also resided in the spiritual realm, under the governance of the Twin Deities. According to Talzin, animal ur-spirits are "red and raw," while plant ur-spirits "pulses with a deep green" and are "quieter and more content." All spirits alongside their physical counterparts comprised Dathomir's "life web."
The Nightsisters further believed that on Dathomir the two realms existed so closely that only their shamans were capable of relaying messages between them. The Nightsister shamans were considered to be "chosen" to interact with the spirit world. They were believed to serve as a conduit for the Spirits themselves.
The lore indicates that the Spirits granted fertility to the clans and claimed the souls of the dead. Nightsister talismans were thought to be imbued with power by the spirits. Darth Sidious considered these artifacts to be similar to those created by the ancient Sith, but no alchemist had ever replicated the unusual effects of the Nightsisters' talismans, like metamorphosis. Sidious found this lack of comparable alchemic power among the Sith to be "curious".
Asajj Ventress, a Nightsister by birth who trained as both a Jedi apprentice and a Sith Acolyte, once noted that her former masters Ky Narec and Dooku did not believe in the Spirits. Indeed, the Jedi viewed the work of the Spirits as a rudimentary name for the Unifying Force and the Living Force. However, the existence of the Twin Deities was confirmed during the Clone Wars, when Jedi Knight Anakin Skywalker was taken to Mortis, a mysterious planet contained within an octahedral monolith.
Numerous parallels existed between the Spirits and the Force. Mother Talzin stated that a Nightsister had to obey the will of the Spirits, much like a Jedi would follow the will of the Force. Furthermore, it was said that Allya prophesied the emergence of a perfect being "brought into existence by the Spirits," which strongly resembled the prophecy of the Chosen One.
The Spirits venerated by the Nightsisters were detailed in the 2012 reference book Book of Sith: Secrets from the Dark Side. Because the section of the book discussing them is written from a Nightsister's point of view, it remains uncertain if the spirits are real or simply primitive expressions of the Force. The book also directly connects the spirits to the "Mortis Trilogy," a story arc from the TV series Star Wars: The Clone Wars.