A Contest of Nisse and Release
The Jade Prison was a massive construct created by the Sidereals at the time of the Usurpation for their Dragon-Blooded allies to use in liberating Creation. The purpose of the Jade Prison was to contain the Exaltations of each Solar as they were killed, so that as they died no new Solars could be incarnated.
All of the souls taken by the Dragon-Blooded at the Great Banquet, as well as the energy of the Solars killed in the first few months of the Usurpation were contained within the Prison. A few souls – at least 17 – were not placed in the prison. 11 escaped in the Invisible Fortress; two escaped in Rathess; and a number of Night Castes fled via their unparalleled stealth. Those Solars who managed to evade the Usurpation for years, or hide themselves in the corners of Creation, escaped this fate. The Wyld Hunt was originally founded to deal with these few left-over Exaltations.
After it was filled, the Jade Prison was sealed with the most powerful magic available to the Sidereals, and then thrown into the Inner Sea and forgotten.
Sometime between the disappearance of the Scarlet Empress and the Invasion of Thorns by the Mask of Winters the Jade Prison was found by agents of the Yozis who had been searching for it. The Yozis formed a pact with the Neverborn and shared this information with them, as well as teaching them to corrupt Solar Essence. In exchange, the Neverborn instructed their Deathlord servants to constuct a massive Bone Warstrider, which was sent to the bottom of the Inner Sea to open the Prison and capture the emerging souls.
The Sidereal protections on the Prison were completely unequipped to resist an attack from a creature that came from outside of Fate, and the Bone Golem easily smashed open the Prison, releasing the souls within. However, the Neverborn did not anticipate the speed and power of the Solar Exaltations, and their creature was only able to capture a small number of them. Forced to obey their pact with the Yozis to turn over 50 Essences (which became the Infernals), the Deathlords converted the rest into the Abyssals.
The remainder of the Exaltations escaped into Creation, allowing a new generation of Solar Exalted to be born.