The release of the Primordial was an unintended consequence of the test-firing of a Senescent Halo, Installation 07, by the Master Builder and Mendicant Bias in the Charum Hakkor system during the Forerunner-Flood War, approximately fifty years prior to the end of the war and the Great Purification. In order to ready the Halo Array for use against the Flood, the Master Builder ordered Mendicant Bias, the AI in charge of the entire Halo Array and much of the Forerunner defense, to test-fire it in order to prove its effectiveness. In addition to causing complete ecological collapse on all planets in the system, it also destroyed any and all Precursor technology, which included the stasis chamber containing the Primordial within the Charum Hakkor arena.
Though motivations are unknown, the Primordial was taken onto the surface of Installation 07, where it was interrogated by Mendicant Bias. The halo then disappeared from Forerunner space for forty-three years, during which time the Primordial convinced Mendicant Bias to turn against its makers and join the Flood’s war effort. This is the first known appearance of the logic plague, something the Flood used against artificial minds to turn them to their side.