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Long long ago, a tale untold by many, before the Era of Raava. Approximate date is unknown, estimate is 50,000 BG.
An island exist, it's size no bigger than 20KM², on it's left side, the Moon half submerged. On the right, the Sun, also half submerged. In front of it, high mountains whence the cold air flows. Behind it the unforgiving desert, wherein none hast ever escaped. On the middle of the island, a peach tree stood tall, underneath it could be seen; a man drinking tea.
Now 30,000 years into the future, the spirit of Light, Raava, duels her brother the spirit of Darkness, thus starting the Harmonic Convergence. Such an event only happens once per 10,000 years, however this time was the only time it was different. Nearing their ten thousand years of battle, Raava and Vaatu appeared before the Sacred Peach Tree, within the Plane of Tawazun. Both were shocked, since none know what is happening.
The man drinking tea calmly explains to them what is going on, and though they were certainly befuddled, they still listened. As they leaves, the words he said plague their minds, until a certain humans broke their fighting. And thus is what the man said;
"Once the Convergence reached it's conclusions, only then shall the world change, and thy shall reveal thyself to the world thou hast preserve. Cities on top of Lion Turtles, humanity becoming the cause of conflict, thy shall be there to watch it unfolds through times untold."
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Water. Earth. Fire. Air.
Long ago, the four nations lived together in harmony.
Then everything changed when the Water Tribe attacked.
Only the Avatar, master of all four elements, could stop them. But when the world needed him most, he vanished.
A hundred years passed, and the Water Tribe continued their relentless conquest. Their powerful waterbenders, with their mastery over ice and the sea, devastated the lands, flooding cities, and freezing entire armies in their path. The Earth Kingdom, though strong and enduring, struggled to resist the unending onslaught of the Water Tribe's forces.
Some people believe that the Avatar was lost forever, taken by the tides of time.
But I believe the Avatar will return to save the world.
1000 years after Avatar Aang era.