Spirit lights

The spirit lights refer to both the Northern Lights and the Southern Lights. These lights are understood to be peaceful spirits that are in balance, dancing and lighting up the sky over the North and South Poles. This phenomenon serves as a reminder to the people of the Water Tribes of their connection to the spirits. If the physical world is out of balance with the Spirit World, as was the case in the Southern Water Tribe, the spirits will not dance in the sky, and the lights will not appear until balance is restored.

History
During the Hundred Year War, the South was devastated by constant raids from the Fire Nation, causing many of their spiritual knowledge and traditions to be lost. The South was then thrown out of balance and the Southern Lights disappeared. After the war had ended, the Northern Water Tribe helped to rebuild their sister tribe physically, but was not able to rebuild them spiritually, and the Southern Water Tribe continued down a path of spiritual decay, most notably by the distortion of the hollowed Glacier Spirits Festival. During this festival, tribal elders used to commune with the spirits and watch the brilliant display of light as the spirits danced in the sky, causing the Southern Lights. The people of the South have since turned the festival into a cheap carnival that celebrates greedy and trivial humans, and they have abandoned all connections with the spirits; therefore, bringing darkness upon themselves.

After the Hundred Year War the Southerners neglect of the spirits had caused the spirits that usually form the Southern Lights to rampage in the Everstorm rather than light up the dark, like the Northern Lights do in the North as Unalaq explains. In 171 ASC, Unalaq informs Avatar Korra of a spirit portal that is located at the South Pole in the middle of a frozen forest, but the portal has long been closed. They both travel to the South Pole with some of Korra's friends and family to try to traverse the Everstorm and open the portal. Korra must go alone into the forest to open the portal if she is to restore balance between the physical world and that of the spirits, but the portal can only be opened during the Winter Solstice.



Korra eventually makes it to the portal, after fighting through the forest and past a number of Dark spirits, where she eventually opens it with a single touch while in the Avatar State. When the portal opens, balance is instantly restored and the balanced spirits flow out in a straight beam of light directly above the South Pole until they disperse in the sky as waving streaks of green, glowing lights. The Southern Lights were finally restored as the spirits danced in the sky. After Korra joins back up with the group, Unalaq proceeds to tell her that restoring the Southern Lights was the first step to bring balance back to the South.