Zeta Halo, formally Installation 07, was one of seven Halos in the Neoteric Halo Array. Its location has never been definitively determined by the UNSC, though it is known to reside somewhere in the Sagittarius Arm of the Milky Way galaxy. Zeta Halo is the only one of the seven Halo’s that was originally part of the original Halo Array, the Senescent Array. This array of twelve halos consisted of rings 30,000 kilometers in diameter, versus the 10,000 kilometers of the Neoteric Halos.
Zeta Halo was test fired many years prior to the Forerunner-Flood War to prove the effectiveness of the halos as a weapons system. During this test on the former ancient human world of Charum Hakkor, the Primordial, an ancient creature thought to be all that was left of the Precursors, was set free onto the ring and interrogated by the ancilla on the ring, Mendicant Bias, with the logic plague. Successfully corrupting Mendicant Bias after forty-three years of communication, the Primordial and Mendicant Bias used the halo array to attack the Forerunner capital Maethrillian. Zeta Halo was the only ring successfully fired on the capital, but it killed everyone in and around the capital world.
Zeta Halo escaped following the attack, and a conflict between Mendicant Bias and the installation itself arose when the facility’s security system tried to crash the halo into a planet. The conflict was resolved with the arrival of the IsoDidact at the installation, where he deactivated Mendicant Bias, killed the Primordial, and piloted the ring-world so that the planet was sent through the hoop itself. The stress of this action on the halo resulted in it shedding large portions of itself, and it was successfully restructured into a hoop 10,000 kilometers in size. Following this event, Installation 07 was repaired and used as one of the seven halos in the Neoteric Halo Array. At the tail end of the Forerunner-Flood War in 97,445 BCE, Zeta Halo was fired for the only time as part of the Great Purification that cleansed the galaxy of all sentient life and eliminated the Flood threat.