The Halo Array, Halos, alternatively known as the Sacred Rings by the Covenant, Fortress Worlds 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.
