Spirit portal

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- It was I who broke through the divide that separated the plane of spirits from the material world!

The Spirit Portals are gateways between the Physical World and the Spirit World that are located at the North Pole in the Northern Water Tribe, the South Pole in the Southern Water Tribe, and downtown Republic City in the United Republic of Nations. The northern and southern portals were created by Vaatu and the Republic City portal was created by Kuvira.

Pre-Era of Raava
Before the beginning of the Era of Raava, Vaatu, the spirit of chaos and darkness, broke through the barriers that separated the Physical World and the Spirit World, creating the northern and southern spirit portals. Over time, spirits freely passed from their world through the spirit portals located at the poles and took residence in the Physical World. However, after Vaatu's defeat at the battle during Harmonic Convergence in 9,829 BG and his subsequent imprisonment in the Tree of Time, Avatar Wan ordered that the spirits return to their world and closed the portals so that no one could physically enter the Spirit World and release Vaatu.

171 AG
During 171 AG, Unalaq tricked Korra into opening the southern portal under the guise of restoring spiritual balance in the Southern Water Tribe. Later, moments before the Harmonic Convergence of 171 AG, he forced Korra into opening the northern portal, after he and his children had failed, by threatening the life of Jinora. Korra later attempted to close the northern portal seconds before Harmonic Convergence to prevent its occurrence, but ultimately failed. The occurrence Harmonic Convergence ultimately freed Vaatu. After merging with Vaatu, Unalaq used the power of the portals during Harmonic Convergence to permanently fuse them together, creating the Dark Avatar. This entity was later defeated by a large spiritual projection of Korra after she retrieved Raava from inside the enlarged Dark Avatar, and was removed from existence through Korra's use of spiritbending. She and Raava later refused using the power of the portals at the closing moments of Harmonic Convergence.

174 AG
During the invasion, the spirit energy cannon was removed from the enormous mecha suit operated by Kuvira and tossed into the Spirit Wilds located in downtown Republic City. This cannon was later found tangled in spirit vines by Kuvira who fired it at Korra in an attempt to kill her. However, the cannon began to draw energy from the spirit vines and as a result, Kuvira was unable to shut if off. It later exploded, with the explosion's energy later imploding and creating a third spirit portal.