Avatar Temple

Avatar Temples are shrines built in honor of the Avatar. These temples seemingly bear a spiritual connection with the Avatar Spirit and all of its past incarnations, and as such are regarded as sacrosanct dwellings.

As the temples are briefly shown, information regarding their purpose and function remains relatively unknown. However, they seem to be frequented by visitors, mostly by those who wish to use the shrines for meditation and concentration.

It has been determined thus far that every nation has at least one Avatar Temple, though Avatar Roku destroyed the Fire Temple while assuming the body of Aang.

Avatar Roku's Recession
Around the winter of 85 BSC, Roku spends five months in the Fire Temple located on Crescent Island, where he begins to try and master the Avatar State with the help of the Fire Sages. Under the guidance of Fire Sage Kaja, Roku tries a variety of meditation exercises in an attempt to develop control over the state. Avatar Roku has trouble concentrating despite his efforts, and exclaims that mastery over the state through meditation would be impossible.

In hopes of speeding the process, Roku decides to use the sun of the Winter Solstice to push him into entering the state. Avatar Roku triggers the state immediately, though he finds himself unable to control his abilities, much to Fire Sage Kaja's displeasure. Without control over the state, Roku ends up destroying the upper portions of the temple and accidentally triggers the eruption of a small volcano before Kaja manages to free him from the Avatar State.

Roku eventually masters the Avatar State and rebuilds the temple pagodas, carving underground passages out of the island's magma underneath the structure.

The Return of Avatar Aang
In the winter of 100 ASC, the newly rediscovered Avatar Aang decides to return to the Southern Air Temple to reunite with the Air Nomads. Upon arriving with Katara and Sokka, Aang is slightly surprised to find the temple desolate, stating to Katara that it was once a lively place bustling with activity.

He dismisses these feelings and eventually arrives at the Air Temple Sanctuary, where he is immediately drawn to the statue of Avatar Roku, who, unbeknownst to him at the time, was his immediate predecessor. While examining the statues of the previous Avatars, Aang and Sokka spot a Winged-Lemur.

Aang, enthralled at the sight of a creature from his culture, sets off to chase the lemur, who leads him to a part of the temple largely affected by the Fire Nation attacks nearly a century ago. Here, he sees the corpse of Monk Gyatso, his nomadic mentor. The sight of his master's remains causes Aang to trigger the Avatar State, and this in turn is instantaneously recognized by the Avatar Temples. Four temples from each elemental faction glow, signaling the return of the Avatar.

The Winter Solstice
Aang, unaware of the true nature of his mission, realizes that he needs to seek the guidance of Avatar Roku. Unsure of how to go about this, he is assisted by Fang, Roku's companion, who leads him in a spirit journey to Crescent Island where he must go, prior to the Winter Solstice's arrival.

Aang arrives at the temple successfully, but is surprised to find that the Fire Sages are attempting to apprehend him. Fire Sage Shyu, the only loyal Fire Sage left, explains that Aang's apparent disappearance prompted the sages to pledge allegiance only to the Fire Lord. The sages do their best to stop Aang from meeting with Roku, though their efforts ultimately fail as Aang is able to enter the sanctuary before the doors permanently close.

Roku explains to Aang that he must stop The War prior to the arrival of Sozin's Comet, by mastering all four bending arts before the summer's end. Aang realizes what he must do, but expresses concern of escaping the temple safely.

Meanwhile, outside, Zhao arrives and readies to apprehend the young Avatar with the help of his soldiers and the Fire Sages. The doors of the sanctuary open to reveal Avatar Roku, who manifested himself in Aang's body, much to the Admiral's surprise. Roku exhibits his Firebending prowess, defeating the opposing forces and freeing Zuko and Team Avatar from the chains that immobilized them. He then destroys the temple permanently through the use of Firebending.

Trivia

 * It is unknown why the temples failed to glow when Aang first enters the Avatar State directly after he is freed from the iceberg (while he is fighting Zuko on his ship). It may be duen to the fact that Aang entered the state by physical trauma and not by meditation, concentration or other emotional or non-physical triggers.
 * In the canonical online game Escape from the Spirit World, the choices for the question "Where did Aang, along with his friends Katara and Sokka, first learn about the past Avatars?" included the Western Water Fortress and the Northeastern Earth Mound. It is assumed that the aforementioned names are the titles of the Water Tribe and Earth Kingdom Avatar Temples, respectively.