|"Angry spirits are already attacking ships in your waters. I only hope we are not too late to change course."|
|— Chief Unalaq to Southern Water Tribesmen.|
The dark spirit incidents were a series of attacks of dark spirits on humans in several places of the world during a time of great imbalance in the Spirit World around 171 AG. By that time, Vaatu's influence on the mortal realm and the Spirit World began to grow exponentially, turning many spirits dark. Most of these dark spirits began to follow Vaatu's orders, and attacked members of the Southern Water Tribe as well as sailors in the South Sea to give Unalaq a chance to open the Southern spirit portal. Vaatu's control was not absolute, however, as many dark spirits went out of control, both hindering Unalaq's plans and attempting to kill Avatar Korra. Shortly before and during the Harmonic Convergence in 171 AG, dark spirits were mobilized en masse by Vaatu to defeat those who threatened his plans. Most dark spirits were dissipated by Korra following Vaatu's defeat, but some remained, enslaved by Hundun to aid his plot to throw the world into chaos. Hundun's forces were eventually purified by Korra, and with Hundun's death, balance was finally restored in the Spirit World.
Vaatu's imprisonment and the Great War
The dark spirit conflicts are part of the more broad conflict between darkness and chaos, represented by Vaatu, and light and order, represented by Raava. After Vaatu was imprisoned in the Tree of Time in 9,829 BG, Wan separated the mortal and Spirit World by the closing of the spirit portals. However, the Dark Spirit plotted his return, and after almost ten thousand years, he began to slowly grow stronger again.
Around 830 BG, an Avatar defeated Hundun and mortally wounded his brother, who were conjoined twin kings and masters of the Chaotic Attack. They retreated to the Spirit World, where Hundun, completely imbalanced and filled with hatred because of his brother's irreparable injury, began to plot his revenge on the Avatar.
In 0 AG, the mortal realm was thrown out of balance by the Hundred Year War. As direct result, spirituality declined rapidly: the Air Nomads, the most spiritual nation, were annihilated, the Fire Nation discarded most of their spirituality, and the Southern tribesmen abandoned many old traditions that were established to appease the spirits of the South Pole in their struggle for survival. As a result, many spiritual areas were tainted and old traditions to appease the spirits were abandoned. Vaatu fed on the chaos and anarchy, growing increasingly stronger.
After the end of the war in 100 AG, the surviving three nations only physically healed the world, causing spiritually unstable places to completely fall out of balance. In case of the South Pole, one of the most important spiritual centers of the world, the southern lights disappeared, while the inhabiting spirits turned dark and roamed the Everstorm.
Spirit attack on the Northern Water Tribe
The spirits of the North fell out of balance in 151 AG, when the spirit forest at the North Pole was destroyed by Tonraq as result of his younger brother's scheme. Angered and subverted into dark spirits, the entities attacked the tribe's capital. The tribesmen were unable to stop the rampaging spirits until Unalaq effectively calmed them by using spiritbending to restore the balance of light and dark within them, sending them back to the Spirit World. As Tonraq's actions had almost caused the destruction of the Northern Water Tribe, his father banished him and made Unalaq the new heir to the Water Tribes. When Unalaq later became chief, he traveled to the South in order to manipulate Korra into opening the Southern spirit portal in hopes to achieve his ultimate ambition of reunifying the mortal and Spirit Worlds, which he believed could be fulfilled by cooperating with Vaatu.
Nearing the Harmonic Convergence of 171 AG, Vaatu's influence grew to the point that he was able to directly influence the Spirit World and the Earth. Beginning in late 170 AG, this caused many spirits to turn dark and those dark spirits who already were in the mortal realm, most notably those in the Everstorm, to openly attack humanity. The number and aggressiveness of dark spirits in the South Pole increased dramatically, giving Chief Unalaq a chance to intervene and open the Southern spirit portal in order to prepare Vaatu's escape.
Attacks in the South
Unalaq addressed his disappointment in the deterioration of Southern traditional practices during the Glacier Spirits Festival, but was largely ignored. The severity of the situation became clear when a dark spirit attacked Korra and destroyed the festival grounds, defeating the Avatar and numerous benders in the process. Unalaq was able to calm the spirit through the use of spiritbending, demonstrating his knowledge of spiritual ways to Korra and prompting her to agree to being his student.
Unalaq assembled a small expedition to the center of South Pole against the protests of Korra's father, Tonraq, and set out to restore spiritual balance in the Southern Water Tribe. While passing through the Everstorm, Unalaq explained to Korra that she had to open a dormant spirit portal at the heart of the frozen spirit forest. The group was attacked shortly after by numerous dark spirits, who destroyed their supplies in the process. This prompted Tonraq to urge the others to turn back once more, but his efforts were rebuffed by Korra, who continued along with the others.
Upon reaching the frozen forest, Korra ventured alone in search of the portal, but was attacked by several dark spirits in the process. Using the Avatar State as a last resort, she managed to touch and open the portal, dispersing the Everstorm and restoring the southern lights, thus calming the spirits in the south.
Stopping Avatar Korra
Although the spirits residing in the spirit forest at the South Pole were calmed, not all dark spirits disappeared, as a giant dark spirit attacked Korra on her way to the Fire Nation while she was being pursued by Desna and Eska on the Mo Ce Sea. At first, Korra simply tried to dodge the spirit's attacks, but the spirit was too fast, and she was knocked into the ocean. She entered the Avatar State in response and attempted to purify the spirit, but her attempt failed, and the spirit proceeded to drag her below the surface of the water. Korra subsequently washed up on an island in the Fire Nation, with no memory of who she was. Found by the Fire Sages, she was brought before a shaman, who determined that the dark spirit had infected her, threatening to destroy her Avatar Spirit. To save her, the Sages lowered Korra into spirit waters, enabling her to reconnect with her past lives and cleanse the infection.
Korra later encountered a swarm of bat-like dark spirits, which burst forth from an airbender meditation circle at the Eastern Air Temple when disturbed by Tenzin's cleansing ritual. The Avatar and Tenzin initially tried to defeat the spirits using regular bending attacks, though when they proved to be ineffective, Korra stepped forward and utilized Unalaq's purification technique successfully for the first time, fully pacifying these dark spirits. As their dark essence left the meditation circle, the environment's connection to the Spirit World was strengthened.
As Harmonic Convergence drew near, the spirits saw an increase in strength and numbers in both the mortal and Spirit World. Due to Unalaq's allegiance with Vaatu, they followed the Chief's orders and helped him to secure the South Pole, both by swatting down the Southern Water Tribe rebels as well as defending the Southern spirit portal. They prevented Team Avatar, Kya, and Tenzin from entering the Spirit World and helped the Northern troops to patrol the defenses. However, when several spirits tried to subdue Bumi, they accidentally destroyed the entire encampment, enabling the team to complete their mission.
When Korra was meditating in the Tree of Time, having bent the energy within herself to create an astral projection of herself to fight her uncle who had merged with Vaatu, a horde of dark spirits charged the tree in an attempt to stop her. They met the resistance of Tenzin, Kya, Bumi, Bolin, Mako, Desna and Eska, and although they suffered some losses, their sheer number eventually overwhelmed the handful of humans. As they were about to swarm the hollow of the tree in which the defenders had retreated, Korra's astral projection returned, dissipating them all.
Despite Vaatu's defeat, not all dark spirits were purified. Many of Vaatu's former servants were subsequently enslaved by Hundun, who also used his powers to turn additional spirits dark. This way, the dark master could amass a huge swarm of dark spirits at the Tree of Time, and used the opened Southern spirit portal to take some of them to the South Pole. There, they confronted Avatar Korra when she tried to pursue Hundun into the Spirit World. After a lengthy battle, the spirits pushed her through the portal, so that Hundun's main force could destroy her. Despite its effort, the dark spirit swarm could not stop Korra from reaching the Tree of Time, however, and many of them were purified by spiritbending. At Vaatu's former prison, Hundun's dark spirits launched their yet biggest attack, and almost overwhelmed Korra until she entered the Avatar State, dissipating most of them. After Hundun lifted the ground they were standing on into the air, his last dark spirits were quickly destroyed by Korra, leaving him to face the Avatar alone.