Disclaimer: I don't know Avatar: The Last Airbender, the opening quote, or the closing quote. The opening quote belongs to Charles de Lint, and the closing one is based off the opening to The Wheel of Time series.
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Once upon a time, there was what there was, and if nothing had happened there would be nothing to tell. -Charles de Lint
Once upon a time...
There was Something. It hadn't always been so, and certainly wouldn't continue forever, but for now, there was Something. This Something would, eons later, be called the Spirit World by those would live upon the mortal plain. Those who lived in the Spirit World were few, but powerful. Extremely powerful, as the power threatened to rip the Spirit World apart and send it back to Nothing. Something has always been born of Nothing, just as Nothing was always born of Something, although no one, not even Koh the Historian and Wan Shi Tong, He Who Knows Ten Thousand Things, knew of this.
A group of spirits, the most powerful, left the Spirit World to bring it to balance. They wandered the cosmos, until they found a suitable place to give the power away. And they found it.
The spirits of the earth gave part of themselves to form what would be known as the Human World, or the mortal plain. They formed other planets, other rocks and meteors and asteroids, but it was unto this one that they gave part of their essence, part of their being. When they had given, they stepped back for others to take their place.
The spirits of fire stepped forth. One traveled the world, heating it and setting it afire. The other scattered his light around it, forming the sun that light the world, and those that light the universe, the other stars, and comets. The one who traveled the earth left the surface and sunk down to the center to form the world's core. When the pure fire at the world's core had come into existence, the spirits of fire left for others to continue their work.
The spirits of air danced upon the world. They put out the fires on the world's surface and blew the molten lava into mountains and valleys, fjords and plains. Unlike the others, they had no intention of leaving. Someone must stay behind to watch over this world they were creating, and who better than the spirits of freedom?
Tui and La, the water spirits, were the last to form the world. La covered the world in water, while Tui formed the moon. Like the two, they pushed and pulled at each other, forming the tides. This revealed the earth once more, and started the great cycle again.
This cycle would become known as the Law of Seasons, as it followed the pattern of spring, summer, fall, and winter. Years later, it would take on a new name, the Avatar cycle.
Even with this, the spirits were not yet done. The excessive power was gone, but all had a compelling need to finish what they had started.
From air and water came storms and rain. From air and fire, lightning. From earth and water came places suitable for living, for those who had yet to come. And the gift of earth and fire: lava from the earth's core to form new land, even as water and air wore away at the mountains. And yet, there was nothing alive.
From earth, water, and the sun's light came all type of plants. From water, earth, air, and the spark of life from fire came fish and land creatures and sky creatures and one who breathed fire, one set for each of the elements. And still, the spirits were not happy. They traveled back to the Spirit World and asked Koh and Wan Shi Tong for guidance. The two talked, and this was their answer: There was no one who could truly appreciate what the spirits had done. The animals were good, but what was the purpose. What they had yet to create were people who could think on the same level as them.
The spirits returned to the planet, and together they created humans. The spirit of water, as before, gave then parts necessary for life, and the spirits of earth gave a shell to contain them and to provide shelter. The spirits of fire gave the spark of life, and humans were born. This time, however, the spirits of air gave what they had never given before: wisdom. The Other World had new guardians now, and so all the spirits returned to the Spirit World.
But all was not well in the mortal world. The humans separated and fought. Some went to the poles, others went to the islands in the west. Some went to the mountains and some stayed on the plains. What the spirits had forgotten was this: they had created the world and other living things with power and essence, so that they knew that something as different as the spirits could be exactly the same, that something apart could be together. Humans were created with only power, and so did not know this. They remained separate, different.
And so the people at the poles looked at the moon, and saw what it did to the oceans. Two lovers on the plains looked at the badger moles and the tunnels that were created. The people on the islands watched the sun rolling through the sky and saw the lava flowing from the earth's core. The people who lived on the mountains saw the Flying Bison, and marveled at it. And that was a beginning, of sorts. They had learned to control the elements that had created them.
The spirits looked at the humans they had created, and what the humans had learned to do. They decided to create one more human, one of spirit, who would walk equally among the people, and among the worlds. This one would not die, for the spirits planned to give him their immortality. This one would continue their work in maintaining balance in the worlds. He would be of the people in the poles, of the plains, of the islands, and of the mountains. He would be man, women, solider, healer, monk, guard. He would know them, be of them, and all would know of him, both mortal and spirit.
And so it was. As those in the poles learned waterbending, as those of the plains learned earthbending, and those on the islands learned firebending and those on the mountains learn airbending, the Avatar was born.
The spirits gave their essence to him, and left the Spirit World forever, as they were too weak to return. Tui and La went to the North Pole, where those who called themselves the Water Tribe lived. The spirits of fire traveled to the center of the world, while the earth spirits took shelter in their element. The wind spirits still dance among the mountains, and if you find them, it is said that they will grant you more knowledge than Wan Shi Tong, for even he cannot find them.
The Avatar was first born into what would be called the Earth Kingdom, where Oma taught him earthbending. He learned the elements in the order of the Law of Seasons, and when he died, he was reborn into the newly formed Fire Nation. And so the Law of Season was renamed as the Avatar cycle.
And years passed. Wan Shi Tong migrated to the human world, and Koh, who was always bored, played a new game with emotions. New spirits, like Enma, were born and the Avatar kept balance. The Avatar was born, died, and was reborn, and, for a time, everything was as it should be. The nations were apart, but together; different but the same. There was war, but there was peace as well.
Only once has the Avatar been dead to the world, and that was these last one hundred years. While he was gone, the spirits revealed themselves, as themselves, for the first time since the Avatar was born.
Although Iroh will never know it, when he sought a way to find the Spirit World to see his son again, it was the spirits of fire that showed him the way. When Huu sought enlightenment on the great tree, it was the earth spirits, not Tui and La, that showed him that everything is connected, and that a waterbender was as much connected to the earth as an earthbender. Tui blessed a newborn girl, and marked her with its spirit. The vision Guru Pathik saw was sent by the air spirits so he would meet Aang when Aang needed training.
Indeed, all these things were arranged by the great spirits, because Aang needed to meet all of these people, and learn something from each one.
Now the Avatar has returned. Now it does not matter if the Fire Nation wins or loses, because there have been other wars, but the Avatar survives. As long as he is not killed in the Avatar State, he will be reborn and all will be well. What once was will be again.
Once upon a time... it began. It was not the beginning, but it was a beginning.
