"After exhausting every other strategic option, my creators activated the rings. They, and all additional sentient life within three radii of the galactic center, died, as planned. Would you like to see the relevant data?" - 343 Guilty Spark
History[]
The Halo Array, or Halos were constructed by the Forerunners a few hundred thousand years after the Flood were discovered by the Array's makers, because their Builder Caste saw their current isolation and containment procedures were not adequate to stop the spread of the Flood. Little is known of their extensive history, but after exhausting every other tactical option, their creators had no choice but to activate the Halos' main weapon, and terminate all sentient life in the galaxy.
When activated, the Halo rings would wipe out all sentient life within three radii of the Milky Way's Galactic center, by sending out radiation, targeting certain cells in the nervous system, which includes, but is not limited to, neurons, by harmonizing all neurological frequencies.
After the rings had been fired, all seven would remain relatively dormant for one hundred thousand years, until one of then was discovered by the theocractic alien superpower, the Covenant Empire. The Covenant, who falsely worship the Forerunners as Gods, believed that Halo's main weapon was actually a source of, quote, "Divine Wind" meant to propel them on a path they called the "Great Journey". The Humans were next to discover this installation, Installation 04, after the Flood was released from containment and began to tighten a grip upon the Ring Word. What remained of the Humans from the UNSC Pillar of Autumn decided to detonate the damaged Nuclear Fusion Reactor and stop the spread of the Flood and to stop the Covenant from firing the ring. That very same year, another Halo Installation was discovered by Humanity, Installation 05, and shortly after discovery, the Flood was once again released and containment measures put in place by Forerunner Automatons failed, letting the Flood run wild on the surface.
When the Human-Covenant War ends, Humanity began to seek out the Halo Installations that were still active in the Galaxy. Their goal being to deservice the rings and ensure that they can't be used to wipe out life within their range of three galactic radii.
Weapons and Tactical Use[]
Canon Reference[]
http://halo.wikia.com/wiki/Halo_Array - Halo Array