The Spirit World is a parallel plane of existence that co-exists along with the World of the Avatar. It is the home of spirits, immortal supernatural entities which often embody different aspects of life and nature. All the past lives of the Avatar exist in the spirit world; it is not known whether other mortals travel here after death in this way.
It is difficult, and therefore rare, for a human to pass into the Spirit World; conversely only a handful of spirits are able to exist physically in the mortal world. The Spirit World and the physical world are closest together during the solstices, which makes it possible for spirits to manifest in the mortal world and for humans to pass into the Spirit World. There are also certain locations in the physical world which act as bridges to the Spirit World, and often these locations are revered by cultures living near them, such as the Spirit Oasis in the North Pole. The Avatar itself also acts as a spiritual bridge between both the worlds and can travel between them freely, though bending has no effect in the Spirit World.
There are few known ways for a human to pass into the Spirit World. Probably the most common way is to be carried there by a spirit as the spirit shifts between planes. The Avatar is also able to pass fully into the spirit world at times and locations where the barrier separating the two realities is unusually weak (such as, at Avatar Temples on a solstice or at the Spirit Oasis at the North Pole).
Very little is known about the Spirit World, in particular its geography. At least in some places, the Spirit World seems to conform closely to an area of the physical world, so much so that a mortal making the transition might not immediately notice the difference. In other places (such as the North Pole) the geography of the Spirit World seems to have no connection to the physical world, with completely different geography. Koh's Realm
Main article: Realm of Koh
The realm of the spirit Koh, the Face Stealer, exists below a gigantic, twisted tree. The land surrounding his lair is barren, dark and scarcely inhabited, except for Koh's faceless victims and at least one gargantuan animal. For anyone who enters his lair, it is advisable to remain emotionless, as Koh steals the face of anyone who expresses his or her emotions in front of him.
Main article: Solstices
A solstice is an astronomical event that occurs twice a year. During these solstices, the Spirit World and the physical world are significantly closer than during the rest of the year. So, it is easier to cross over during these solstices. Aang used the Winter Solstice to talk to Avatar Roku about Sozin's Comet.
Main article: The Avatar
The Avatar is the natural 'bridge' between the Spirit World and the physical world. The Avatar can use meditation to enter the Spirit World. When he/she does this, the spirit of the Avatar leaves his/her body, leaving their body defenseless.
The Spirit Oasis is an oasis at the Northern Water Tribe, behind the chief's temple, at the base of a waterfall. This place is the center of all the spiritual energy in the entire North Pole, and unlike the rest of the North Pole, it is noticeably warm there. In the middle of the Spirit Oasis are two koi fish. The two fish are actually the mortal forms of Tui and La, the Moon and the Ocean spirits, circling each other in eternal dance.
Koh, the Face Stealer: An ancient, malicious spirit who steals the face of anything which shows emotion. He is extremely knowledgeable, being one of the oldest spirits in existence, and can also see into the future. Avatar Kuruk holds a grudge against him, constantly hunting after Koh for taking his loved one, Ummi.
Tui andLa, the Moon and Ocean Spirits: Near the beginning of the world's creation the Moon and Ocean spirits departed for the mortal world and took the form of two koi fish in the sacred Spirit Oasis of the North Pole. Tui and La were the original Waterbenders with La being the source and Tui being the originator.
PrincessYue, the second Moon Spirit: After Tui was killed by AdmiralZhao, Princess Yue sacrificed her life to become the second Moon spirit, which gave her Waterbending abilities. Since her father's bargaining with Tui she has been the reincarnation of the original Moon spirit.
Hei Bai, a Forest Spirit: Takes the form of a panda bear but its real form is a many armed monster. Spirit of a dense forest near Senlin Village, Hei Bai attacked the village when the Fire Nation destroyed his forest. He later helps Aang several times in the Spirit World.
Wan Shi Tong, the Knowledge Spirit: Wan Shi Tong brought his immense library of knowledge to the mortal world in the hopes of bettering mankind with wisdom. Uses fox-like animals known as Knowledge Seekers to help him with his discoveries. The library was located in the Si Wong Desert. When the mortals instead used the knowledge to further warfare, Wan Shi Tong buried the library beneath the sand so none may ever find it again. Takes the shape of an owl but when angered becomes a wyvern like creature.
Painted Lady: Spirit of the Jang Hui River, in the Fire Nation, where villagers worship and have statues of her. She thanks Katara for filling in for her after she fled the village because of the river's pollution. The Painted Lady has been known to have healing and spiritual abilities.
Enma: A cranky monkey-like spirit that spends his time meditating.
Avatar Aang: An Air Nomad Avatar and the second most recent. Aang was the last Air Nomad and helped stop The War. After The War, Aang married the Water Tribe girl, Katara, and together they had a son, Tenzin.
Avatar Roku: A Fire Nation Avatar that is a guide and mentor to Aang throughout his journeys. He was once the best friend of Sozin, but they later became enemies. Sozin left him to die from the poisonous gas from a volcanic explosion. His not killing Sozin was a major cause of the War. He was also a friend of Monk Gyatso.
Avatar Kyoshi: An Earth Kingdom Avatar from Kyoshi Island that was a tough and a "by any means necessary" type of Avatar. She founded the Dai Li to ensure protection of Ba Sing Se's cultural heritage. She has dealt with the likes of the 46th Earth King and Chin the Great, the latter of which she killed.
Avatar Kuruk: A Water Tribe Avatar from the Northern Water Tribe that constantly hunts Koh. He was a laid-back and easy-going Avatar that flirted with girls and liked to show off his bending prowess. This would eventually be his downfall in life.
Avatar Yangchen: A female Air Nomad Avatar from the Western Air Temple that advises Aang in the series. She was willing to forsake her spiritual customs to ensure peace and safety in the world.
Unnamed Fire Avatar: Fire Avatar that was shown erupting volcanoes while in the Avatar State.
Fang: Roku's dragon that died along with Roku during the volcano explosion. He can now communicate telepathically with people from the mortal world and give them insight into the Spirit World. As a spirit, Fang is intangible and invisible.
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