Halo doesn't kill Flood. It kills their food. Humans, Covenant, whatever. We're all equally edible. The only way to stop the Flood is to starve them to death. And that's exactly what Halo is designed to do: wipe the galaxy clean of all sentient life. — Cortana The Halo Array, Halos,[1] alternatively known as the Sacred Rings[2][3] by the Covenant, Fortress Worlds[4] by their creators, Installations by the A.I. Monitors that run them, and the Great Wheels by prehistoric humans, are seven (of an original twelve) massive ring-shaped installations constructed by the Forerunners throughout the galaxy over one hundred thousand years ago as a last resort when combating the parasite known as the Flood. When nothing else could be done, they activated the rings which killed all sentient life forms within three radii of the galactic center. Not only are they weapons of last resort, but the Halo installations are also research facilities, mainly for the study of the Flood.