History of the Avatar

Introduction

The Avatar 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 and peace 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 Avatar is to master the elements, beginning with the bending art of the Avatar's birth nation.
As part spirit, the Avatar possesses an innate connection to the Spirit World and 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.
The Avatar posses other powers besides being able to bend all four elements such as being capable of recalling memories from past lives and 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 realized".

History

The Avatar has been an ever present force through out history, being thousands of years old. No one knows how the cycle began, with some saying that the cycle has always been present as long as humans have been around.
A new Avatar is born within a short period of the previous avatar dying, this can range from days to months and even a year. The Avatar grows up to learn to master every element and become the protectors of the world maintaining peace and harmony within the Four Nations. 
Over the millennium there have been thousands of Avatars, most people only know of four avatars. The current avatar and the previous three. As time goes on the deeds of the previous Avatar's slowly fade away into books rather than the memories of men. Generations are named after the Avatar alive at the time of birth, those born within transitional times as a new Avatar grows are known as Spirits until the name of the Avatar is discovered.

Avatar Cycle

When an Avatar dies, the next Avatar will be born into the next nation in the Avatar Cycle.
The Avatar Cycle also determines the order in which each Avatar is supposed to learn the four bending disciplines. The nation into which the Avatar is born identifies the first element that is to be mastered, often prior to the bender being revealed as the Avatar. For instance, an Avatar born into the Water Tribe first learns waterbending, before attempting to master the three remaining bending disciplines in the order of the cycle: earthbending, firebending, and finally airbending.
Each Avatar has a particular element with which a low affinity is established, dictated by the personality of that Avatar. This element can be difficult to learn, sometimes proving impossible to control until faced with a situation in which the Avatar is forced to reach within and manifest the power.

Identifying the Avatar

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, known as the Avatar relics. 
Earth Avatars were traditionally identified by directional geomancy, a series of rituals designed to winnow through the largest and most populous of the four nations as efficiently as possible. A special set of bone trigrams was cast and interpreted by the earthbending masters, ruling out half the Earth Kingdom as the location of the newborn Avatar. This ritual was repeated in the remaining territory, with the possible locations shrinking by halves each time until the searchers were brought to the doorstep of the Earth Avatar child. 
The methods used by each nation for identifying the next Avatar are not unfailing.