Avatar

"The Avatar, formed initially when Wan permanently fused with Raava during the Harmonic Convergence of 9,829 BG, is the human embodiment of light and peace through the connection with the Avatar Spirit. As the only physical being with the ability to bend all four elements, it is considered the Avatar's duty to master the four bending arts and use that power to keep balance among the four nations of the world, as well as between mankind and spirits. Upon death, the Avatar Spirit causes the Avatar to reincarnate into the next 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 is to master the elements, beginning with the bending art of the Avatar's birth nation. The reincarnation cycle can only be broken if the Avatar is killed while in the Avatar State, Raava is removed from the 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 Worldand is at their strongest in that realm, due to being surrounded by spiritual energy. The Avatar used that connection to the Spirit World to be the bridge between the two worlds in order to keep peace between them and ensure harmony. After the Harmonic Convergence in 171 AG, however, Avatar Korra left the spirit portals open and renounced her role as the bridge between the two worlds to let humans and spirits live together in harmony. The Avatar is also capable of recalling memories from past lives. This ability, however, can be injured or even completely severed. [...]" "Each nation has a method of recognizing the new Avatar. Among the Air Nomads, this involved presenting infants with a selection of toys and watching for a child that selected the exact four that had belonged to past Avatars, called the Avatar relics. Traditionally, the child is told of their identity as the Avatar on their sixteenth birthday. [...]""The Avatar also possesses the ability to bend energy itself, although few Avatars have learned this technique, and even fewer have used it. [...]""Upon mastering control of the Avatar State, after having learned to bend the four elements, an Avatar is referred to as 'fully realised'."