A Watchful Eye
Towards the end of the First Age the corruption and depredation of the Solar Exalted became increasingly too much to bear. The Unconquered Sun had already turned his face from his chosen and the Sidereal Exalted had received the Great Prophecy, and feared destruction. They aided and abetted the Dragon-Blooded into plotting to other-throw their Solar masters.
During Calibration, when almost all of the Solars were gathered in Meru at the yearly feast (an attempt to stop anyone summoning Third Circle Demons), the Terrestrials attacked, killing dozens outright with carefully laid traps and potent weaponry. The few that survived fled, and were hunted down across the next fifty years. The most powerful Solars all perished in the original attack, as they were chosen as the primary targets – first to fall was Queen Merela who was poisoned by her own attendant, and then slaughtered whilst weakened.
The Sidereals meanwhile had been constructing an elaborate device intended to capture the Solar Exaltations and stop them from reincarnating into others. Known as the Jade Prison, this construction was extremely successful, and captured almost all of the solar exaltations perfectly within it. It was then sunk to the depths of the ocean to hide it from the outside world.
To hide their place in the conspiracy, the Sidereals invoked the Mask more powerfully than ever before, accidentally shattering it as they did. This no only hid their involvement, but partially severed them from fate, making it impossible to remain prominent in the minds of mortals from that moment onwards.
The Usurpation was the catalyst which became the The Age of Sorrows.