The Avatar is the only physical being with the ability to learn all four bending disciplines. By holding the Avatar Spirit, the Avatar is the human embodiment of light and peace.It is considered the Avatar's duty to master the four elements and use such power to keep balance among the four nations of the world, as well as between mankind and spirits. When the Avatar dies, he or she is reborn into the following nation, dictated by the cyclic order; fire, air, water, and earth. Mirroring the order in which the first Avatar, Wan, learned the elements, this is also the traditional order in which any one incarnation of the Avatar is to master all four elements, beginning with the element of their birth nation. The reincarnation cycle can only be broken if the Avatar is killed while in the Avatar State, Raava is removed from the current Avatar and destroyed, or the Avatar Spirit is compromised by a spiritual infection.As part spirit, the Avatar possesses an innate connection to the Spirit World and uses it to be the bridge between the two worlds, to reconcile them and ensure harmony. The Avatar is also capable of using this ability to connect with their memories of their past lives. Throughout the ages, countless incarnations of the Avatar have served to maintain harmony in the world. Each nation has a method of recognizing the new Avatar. Among the Air Nomads, this involved presenting infants with a selection of toys to play with and watching for a child that selected four specific toys that had belonged to past Avatars, called the four Avatar relics. Traditionally, the child is told of his identity as the Avatar on his sixteenth birthday. However, Avatar Aang was told much earlier than this because of the likeliness of a war starting due to the Fire Nation's growing aggression,and Korra was discovered as the Avatar when she was four years old, when she exhibited bending abilities beyond her native waterbending without formal training. The Avatar also possesses the ability to bend energy itself, though very few Avatars have learned this technique, and even fewer have used it. Avatar Aang learned this technique from the last living lion turtle and used it to remove Phoenix King Ozai's firebending, thereby ending the Hundred Year War, and again to end Yakone's reign of terror in Republic City by stripping him of his waterbending. Korra was also taught this ability after being able to contact Aang when she lost her bending. Aang used this ability to restore her bending and she likewise did the same for Lin Beifong and many other victims of Amon's unique technique. When the Avatar masters control of the Avatar State, they are commonly referred to as a "Fully Realized Avatar".
Before Wan became the first Avatar, he was banished into the Spirit Wilds from the fire-bestowing lion turtle city after he stole the power of firebending. However, for two years since his banishment, Wan had befriended the spirits, honed his firebending skills, and decided to travel the world to find the other lion turtle cities. During his journey, Wan encountered Raava, the spirit of light, fighting against Vaatu, the spirit of darkness. Unaware of Vaatu's identity at the time, Wan was tricked by the dark spirit into separating him from Raava, which in the process, releasing chaos into the world. Wishing to fix his mistake, Wan journeyed with Raava to search of other lion turtles for the purpose of gaining the elements of air, water, and earth, and learning how to master them. A year later, during their fight against Vaatu at the time of the Harmonic Convergence, Wan merged permanently with Raava, becoming the first Avatar and gaining the ability to bend all four elements at once. After he sealed away the dark spirit, Wan subsequently oversaw the departure of the spirits to their own world, before closing the portals to the Spirit World, ensuring no human would be able to physically travel there and risk Vaatu's escape. Wan took on the task of being the bridge between the two worlds, maintaining balance and peace. Years later, Raava assured a dying elderly Wan that they would be together throughout all his lifetimes, heralding the beginning of the Avatar Cycle and ensuring the continuation of the Avatar's role as peacekeeper of both worlds
The reincarnation of the avatar is controlled by the Avatar Cycle of the elements: air, water, earth, fire. The cycle is based on the passage of the four seasons (air is autumn, water is winter, spring is earth, and fire is summer).When an Avatar dies, the next Avatar will be born into the next nation in the Avatar Cycle, for example, Avatar Yangchen from the Air Nomads was Followed by Avatar Kuruk from the Northern Water Tribe, who was Followed by Avatar Kyoshi from the Earth Kingdom , who was Followed by Avatar of the Year from the Fire Nation, who was Followed by Avatar Aang from the Air Nomads, who was Followed by Avatar Korra from the Southern Water Tribe. ==Avatar State== The strongest and most powerful ability That the Avatar can invoke is the Avatar State . It was created when Wan Permanently bonded with rAAV by touching the energy coming through the spirit portal during the Harmonic Convergence in 9.829 BSC . It allows the Avatar is a vast cosmic energies channel and the knowledge of previous Avatars , Enabling them is especially powerful and perform extraordinary feats bending . The Avatar april unconsciously enter the state in response to perception of mortal danger , Utilizing the state as a defense mechanism , or emotional distress . While in the state , the Avatar has access bending techniques that they themselves have a small not yet learned , but have no conscious control over Their actions and can cause great collateral damage . An Avatar with full mastery of the Avatar State has conscious control over the state and can enter and exit it at will . For an avatar with complete mastery , Their Eyes will glow momentarily , not a continuously . It was by entering the Avatar State Aang reflexively That you are able freeze himself and Appa inside an iceberg for a hundred years by using a combination of airbending and waterbending . He did not physically age at all during the century , as he was in suspended animation, but it took a great physical toll on his body after being in the Avatar State for an extended dry period of time .