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.After deciding to keep the spirit portals open, Korra declared that the Avatar would no longer be the bridge between worlds, but she would continue to use Raava's light spirit to guide the world toward peace.The Avatar Cycle is based on the passage of the four seasons, which coincides with the order in which Wan first learned the elements; fire related to summer, air to autumn, water to winter, and earth to spring.As a means of connecting with the cosmic energy of the universe, Korra meditated in the Tree of Time and was able to bend her own energy to release an astral projection of herself to battle Unalaq.With help from Jinora, who sped up Raava's growth within Vaatu, Korra managed to extract the light spirit from her dark counterpart and defeat Unalaq with his own pacification technique, purifying and dissipating the Dark Avatar.After he sealed away the dark spirit in the Tree of Time, Wan oversaw the departure of the spirits to their own world and closed the two portals to the Spirit World, so that no human would be able to physically travel there and risk Vaatu's escape.
The Avatar Cycle continued for ten thousand years, until the next Harmonic Convergence, when Avatar Korra battled Unalaq, who attempted to create a new world under Vaatu's influence by fusing with the spirit to become the Dark Avatar.
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 World and is at their strongest in that realm, due to being surrounded by spiritual energy.
Before Wan became the first Avatar, he was banished into the Spirit Wilds from the fire lion turtle after he had stolen the ability of firebending.
Over the course of the two years following his banishment, Wan befriended the spirits, honed his firebending skills, and decided to travel the world to find the other lion turtle cities.