|"There are no evil spirits, there is light and dark in them all. But when they're unbalanced, the darkness takes over."|
|— Unalaq explains the dark spirits to Korra.|
Dark spirits are entities who have become unbalanced due to the influence of the spirit of darkness and chaos, Vaatu. Despite Vaatu's imprisonment within the Tree of Time by Avatar Wan, spirits were still susceptible to turning dark depending on the state of the environments they considered important. Particularly, the Water Tribes experienced several incidences of dark spirit attacks following the Hundred Year War, due to the destruction of a sacred spiritual forest in the North Pole and progressive spiritual neglect in the South Pole. Dark spirits attack seemingly at random, with many lurking in sacred or secluded areas that have become neglected over time.
Inhabitance of the Spirit Wilds
In 9,830 BG, Wan unknowingly freed Vaatu from Raava's grasp, thereby allowing him to roam the Spirit Wilds and use his influence to turn other spirits dark, with the intention of harnessing their anger into strength for use in his battle with Raava during Harmonic Convergence. Wan managed to stop him from causing further transformations by merging with Raava and becoming the first Avatar, after which he imprisoned Vaatu inside the Tree of Time in the Spirit World. This allowed the dark spirits whom Vaatu had converted to return to a balanced state. As a means of eliminating conflict between humans and spirits, Wan also sealed the spirit portals and oversaw the return of the spirits to the Spirit World.
Despite these actions, some spirits still managed to cross over to the mortal realm periodically. For instance, dark spirits began to appear in the Fire Islands shortly after the death of the mothers from a village under the control of Toz. With the sole intent of haunting and punishing the warlords in the Fire Islands, the spirits drifted into encampments during nighttime and abducted children. The dark spirits ceased their actions after the first Fire Lord united the Fire Islands and brought the warlords to justice. They eventually became known as the Kemurikage and eventually became part of Fire Nation legend.
Hundred Year War
The Hundred Year War contributed to the resurgence of dark spirits, as the global conflict caused several spirits to turn dark. Following the Air Nomad Genocide, dark spirits began to inhabit deserted areas that previously had spiritual significance to the Air Nomads, including areas of meditation in forests located near the air temples.
In 99 AG, destruction of the forest near Senlin Village also caused Hei Bai to turn dark. Having protected the forest for thousands of years and unaware that the Fire Nation was responsible for its destruction, Hei Bai terrorized the nearby village as the winter solstice drew near, during which time the separation between the two realms was minimal. As punishment for the forest's destruction, Hei Bai raided the village every night and abducted a single villager during each of his attacks. He eventually reverted to his docile form after Avatar Aang ensured the renewal of the forest by showing him an acorn.
Attacks in the Water Tribes
Following the Hundred Year War, dark spirits also appeared in the Water Tribes due to spiritual imbalances. In 151 AG, many spirits were angered after Tonraq destroyed a sacred spiritual forest in the North Pole while in pursuit of barbarians who attacked the Northern Water Tribe. As a result, they rampaged through the capital and destroyed infrastructure until they were pacified by Unalaq using spiritbending. This technique restored the balance of light and dark within the spirits, and they subsequently returned to the forest.
Due to increasing neglect of spiritual traditions, spiritual imbalance also became characteristic of the Southern Water Tribe following the end of the Hundred Year War. This resulted in the dissipation of the southern lights, since the spirits ceased to dance in the skies. Instead, the spirits turned dark and rampaged in the South Pole in the form of a torrential blizzard known as the Everstorm. Dark spirits roamed within the storm and inhabited the harsh tundras, attacking individuals who attempted to journey through the storm. The phenomenon persisted for decades until Avatar Korra and a small expedition team set out to disperse it by reopening a dormant spirit portal at the heart of the South Pole. Dark spirits also lurked in the seas surrounding the region, as there were many incidents of attacks on ships in the waters near the Southern Water Tribe.
Resurgence during Harmonic Convergence
Dark spirit incidents increased as Harmonic Convergence neared. A dark spirit assaulted Korra on the Mo Ce Sea and was able to infect her, posing a lethal threat to the Avatar Spirit. Shortly thereafter, Korra and her group were also attacked by spirits that inhabited an ancient meditation area near the Eastern Air Temple, having been released when Tenzin attempted a spiritual cleansing. They were resilient to bending attacks, but were pacified by Korra when she performed spiritbending. As the spirits dispersed, their dark energy was also cleansed from the surrounding stones, restoring the environment from a dilapidated state.
Several naturally dark spirits inhabited the Spirit World, including a dark spider spirit which trapped intruders in a thick web before tossing them into the Fog of Lost Souls. However, Unalaq's increasing presence in the Spirit World caused drastic changes to the landscape and increased the number of dark spirits present in the realm. The combined negative energies of Unalaq and Vaatu allowed the former to amass an army of dark spirits to fight alongside the Northern Water Tribe military for the remainder of the Water Tribe Civil War, aiding Unalaq's forces in overpowering the Southern Water Tribe rebels and defending the Southern spirit portal. During Harmonic Convergence, the horde of dark spirits also attempted to prevent Korra from defeating the Dark Avatar. They dissipated following the return of Korra's astral projection to the Tree of Time.
The dark spirits are seemingly invincible, possessing chaotic and nearly amorphous bodies that sustain damage with limited to no effect and enable them to stretch their limbs and form additional appendages. Spirits are capable of inflicting significant damage to individuals or infrastructure, and can enter machinery in order to tear it apart from the interior due to the intangibility provided by their body density.
Spirits can appear and dissipate at will, with the ability to assume different forms depending on the situation; they can contort and fuse together to become larger and more powerful spirits or shift into more convenient forms. They move at varying speeds, inflict damage to different degrees, and are affected by stimuli in different ways, reflecting the presence of individual characteristics and weaknesses despite overarching general similarities.
Dark spirits are impervious to most forms of bending, but are susceptible to spiritbending when performed correctly. Like other spirits, they are also capable of possessing humans, thereby subjecting the host to mortal danger. In the case of the Avatar, dark spirit possession can result in the endangerment of the Avatar Spirit, as the invasive dark spirit feeds on Raava's energy.
- In "The Southern Lights", the lurking spirits that followed Korra and her group throughout the South Pole were loosely based on African wild dogs, while the serpent-like spirits that attacked her in the sacred forest were based on pelican eels.
- There is a special relationship between light and dark spirits, as one cannot exist without the other.
- Dark spirits are stated by Unalaq to not be inherently malevolent, but rather chaotic, merely reacting to nature's destruction. This, combined with their supposedly intricate relationship with light spirits, reflects traditional Asian ideas of yin and yang, that neither darkness nor light have moral attributes, and that evil comes from imbalance. When spirits are angered, Vaatu takes advantage of this and corrupts them with his essence.
- Hundun is the only known being besides Vaatu who can turn spirits dark.
- The Kemurikage are the only known dark spirits to have been humans prior to transcendence into the Spirit World.