Dark Spirits (Legend of Korra)

"We are the protectors of the forest! We won't let you burn it down! Wipe them out!"

- Aye-Aye as a Dark Spirit to a group of firebenders before ordering his fellows to slaughter them. 

The Dark Spirits are a malevolent sub-group of the Spirits from the world of Avatar: The Last Airbender and Avatar: The Legend of Korra, first appearing in the second season of the latter show. They are the second season's main antagonist group.

History
The Dark Spirits first appear in the first episode of Book 2. According to the head of the Northern Water Tribe Unalaq, the Dark Spirits are Spirits that have become corrupted by negative energy, which turns them into angry, vengeful, and malevolent entities. Unalaq stresses that the Dark Spirits are not truly evil but rather not themselves, and can be purified so as to return to their original forms.

The first known appearance of the Dark Spirits was twenty years before the events of the present day, when Tonraq destroyed a sacred forest while in pursuit of raiders. In retaliation for this sacrelige, Dark Spirits attacked the Northern Water Tribe and caused massive devastation before Unalaq was able to calm them. Following this Tonraq was banished from the Northern Water Tribe, allowing Unalaq to become chief.

The Dark Spirits returned twenty years later, first attacking Water Tribe fishing ships. Another later attacks the Southern Water Tribe itself on the night of the Glacial Spirits Festival. It proves to be a formidable opponent for Korra, Tenzin, and Tonraq, until Unalaq is able to calm it using spiritual techniques that turns it back into a peaceful Light Spirit. Following this Korra agrees to join Unalaq so as to learn how to better handle Dark Spirits in the future. Unalaq tells Korra that the first order of business is to unlock a spiritual portal and in so doing bring back the Southern Lights, which would calm the Spirits for a time.

After braving a ruined oasis filled with Dark Spirits Korra succeeded in bringing back the Southern Lights, but Unalaq cautioned that this would not be the last of the Dark Spirits and that there was still work to be done. Indeed, while fleeing from Unalaq's children Eska and Desna, Korra comes under attack from a particularly large Dark Spirit that manages to drag her underwater.

It is subsequently revealed in the "Beginnings" two-parter that the Dark Spirits are Spirits that were originally corrupted by Vaatu, who took advantage of certain Spirits anger towards humanity to make corrupting them easier.

Dark Spirits later show up serving Unalaq in the Southern Water Tribe, and are used by the Northern Tribe as shock-troopers. When Korra battles the now Vaatu-possessed Unalaq for the fate of the world, a swarm of Dark Spirits try to destroy Korra's vulnerable body but are kept at bay by Korra's friends long enough for Korra to triumph over Vaatu and Unalaq, with the latter being destroyed. It is assumed that the Dark Spirits returned to their light selves without Vaatu there to corrupt them.

Trivia

 * Grey DeLisle, the voice actress of Azula, has confirmed that she will be voicing one of the Dark Spirits. Perhaps fittingly, one of the Dark Spirits is also confirmed to be pyrotic.
 * Unalaq's claim that Dark Spirits are not truly evil but rather the result of imbalance is similar to Taoist beliefs regarding the ideas of light and dark and good and evil.