The Great Prophecy was a prediction made by a conclave of all the Sidereal Exalted that set the course of the Solar Exalted's murder at the hands of the Dragon-Blooded. Concerned with the Solars' increasingly erratic behavior, the Sidereals prophesied three possible futures.
In one future, they did nothing, and the world was destroyed.
In another, they tried to guide the Solar Exalted back onto the path of righteousness and had a small chance of success, though the world would become a place of darkness and misery if they failed.
In the third, they eliminated the Solars and the world continued on, diminished but whole.
It was decided by the Sidereals that this last future was the safest bet to ensure Creation's survival, so they conspired with the Terrestrial Exalted to bring it about.