Dark spirits

The dark spirits are entities that grew restless by 171 ASC, and managed to cross from the Spirit World to the mortal world. The spirits reside in the icy tundra deep beneath the South Pole, and will encounter Avatar Korra.

History
By 171 ASC, the dark spirits began scourging the icy regions of the South Pole, where they were known to attack ships in the Southern Water Tribe.

It was only six months after the conclusion of the Anti-bending Revolution that Avatar Korra got wind of these attacks, when she visited the Glacial Spirits Festival at the Southern Water Tribe. Her uncle, Unalaq, theorized that the displeasure of the spirits was caused due to the spiritual deterioration and the decay of traditions of the southern tribe.

During Korra's visit, one of these spirits visited the festival at night. The entity charged at the Avatar and managed to pin her down, though was distracted when Mako and Bolin came to her rescue. The brothers proved to be no match for the spirit, and it quickly defeated them. However, by that time, Tonraq and Tenzin had joined the scene and were battling the spirit each in their own way; Tonraq assisted his daughter in attacking the being, though was batted away, while Tenzin attempted to reason with the entity, though he suffered the same fate. The spirit is eventually faced by Korra in the Avatar State, but even that extra power boost, proved to be ineffective against its prowess. It used one of its tendrils to bring Korra to the ground and was about to deliver the final blow, when Unalaq intervened, encircling the being with water. The spirit changed color and started to glow, eventually disappearing.

Trivia

 * Grey DeLisle, Azula's voice actress, will voice one of these dark spirits.
 * There is a special relationship between light and dark spirits, as one cannot exist without the other.
 * At least one of this dark spirits possesses the ability to manipulate fire.