Dark Spirits (Legend of Korra)

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 antagonists.

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 in so doing bring back the Southern Lights, which would calm the Spirits for a time.

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.