Halo Array

Halo is a name given to a set of ringworld superweapons forged by the Forerunners to stop the Flood parasite. In tandem, these "Fortress Worlds" comprise the Halo Array, a galactic network of seven rings designed to sterilize the entire Milky Way galaxy of sentient life, depriving the Flood of hosts and starving it to death. The Halos were constructed at the Ark or Installation 00, a megastructure located far above the galactic plane out of range of the superweapons' pulse. Following the firing of the rings, biological samples kept at the Ark were used to repopulate the species of the galaxy indexed by the Forerunners as part of their Conservation Measure.

As a point of contrast to their destructive main function, the inner surface of each Halo ring houses a self-sustaining ecosystem complete with artificial terrain, designed to both house and study numerous forms of life. Coupled with Flood research laboratories, it was hoped that by studying the effects of the Flood in a controlled environment, a cure for the parasite may one day be found.

The Covenant revered the Halos as the "Sacred Rings". According to the Covenant religion, based largely on oft-misinterpreted, fragmentary or corrupted Forerunner records, the Forerunners had embarked on a "Great Journey", propelling themselves into divinity and leaving behind material existence. By discovering and activating the rings, the Covenant believed they could follow in the Forerunners' footsteps and ascend into godhood.