Wars of the Threshold
At the height of the first age creation was a much larger and more advanced than it is in the Age of Sorrow. The Solar Princes of the Earth spent millennia expanding its borders and building its infrastructure. Then… the Usurpation when the lowest of the exalted rose against their betters and bound their souls into the Jade Prison. The Solar Exalted were imprisoned, the Lunar Exalted fled to exile in the border marches. The sidereals obfuscated themselves from creation and the world fell to an age of ignorance and sorrow.
Now, the prison is broken and the Solars are reborn once more.