Charum Hakkor arena
Toward the end of the human-Forerunner wars, circa 106,538 BCE, the Primordial was discovered by Yprin Yprikushma on a small planetoid near the edge of the galaxy, hibernating inside a stasis capsule. It was then transported to Charum Hakkor thousands of meters beneath the arena. There human researchers found a way to revive the creature. At a later date, the captive's prison was sealed with a timelock as a further precautionary measure.
Following the end of the Human-Forerunner War, the Didact gained access to the prisoner. Upon conversing with the creature, the Didact discovered that it was — or claimed to be — the last Precursor, and that the Forerunners had rebelled against the Precursors. The Didact resealed the Primordial's prison and revealed this information to no one except the Librarian.
Following his revival in 97,445 BCE, the Didact, along with the Manipular Bornstellar Makes Eternal Lasting and his human companions Chakas and Riser, returned to Charum Hakkor. They explored the destroyed ruins of the arena and found that the Primordial had been unleashed from its stasis capsule. The visit also helped to awaken personality imprints of Forthencho, Lord of Admirals and Yprin Yprikushma inside of Chakas and Riser respectively.
A preliminary sketch of the Charum Hakkor arena drawn by Greg Bear.