Halo Array

The Halo Array, alternatively known as the Sacred Rings by the Covenant, Fortress Worlds by their creators, Installations by the A.G.I. Monitors that run them, and Great Wheels by the Innusannon and the Protheans, are seven (originally there were 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.

Background
The Halo Array is a network of seven ring-shaped artificial worlds created by the Forerunners in order to kill all sentient life within range of the array, virtually the entire galaxy. Rather than a weapon of war, these were used as a last resort against the Flood, a parasitic extra-galactic species, originally made by the catalyst as an army of "pawns" to gather genetic information throughout the galaxy to ensure the mantle of responsibility stays within leviathan hands, that threatened to infest the forerunners, to allow the reapers to kill every space-faring lifeform in the galaxy. By activating the rings, the Forerunners denied the Flood access to all sapient lifeforms, falsely believed to be the Flood's infection target, effectively starving the species to virtual extinction, though it returned to dark space, and some smaples clung to life in a number of Forerunner facilities, including some Halo installations. Though separated by thousands of light-years, the seven installations are networked together and capable of remote activation at Installations 00 or 10, both located outside the galaxy. Each Halo installation has a maximum effective range of 25,000 light-years. The exact means the array uses to conduct this "mass sterilization protocol" are unknown, though it is assumed to utilize energy in some type of form.

The Halo Array in its entirety has been fired once in known history, approximately 100,000 years ago, by the Didact in order to stop the Flood from overwhelming the galaxy. Despite being the network's builders, the Forerunners were killed in this last suicidal attempt to allegedly save the rest of the universe from a worse fate. However, four activations of a single halo on a limited power setting have also been recorded. Once at Faun Hakkor, Charum Hakkor, the San 'Shyuum quarantine system, and a possible activation at the Ecumene Capital (Which resulted in the destruction of 10 of the original 12 Halos, the two surviving being Installations 12 and 07). The array has nearly been activated since; Sovereign attempted to use it to pull the reapers from dark space to the milky way proper. None of these attempts were successful.

Looking back to a mere 300 years after the Flood had entered the galaxy, the Forerunner Council granted the Master Builder, Faber, permission to begin redesign the incomplete Halo Array, originally planned as a communication and transportation portal system, similarly to the mass relays. Meanwhile, a Lifeworker called the Librarian, began collecting samples from every species across the galaxy; ensuring that if all means of stopping the Flood failed and the Halos were forced into activation, that the galaxy could still be reseeded with life. The Lifeworker's husband, the Didact, a Promethean warrior, created Shield Worlds such as the planet Onyx, to further ensure their survival. At the Array's completion, twelve Halos had been constructed. However, in a sudden cataclysmic event that was mysteriously undertaken by a Metarchy ancilla (a monitor) named Mendicant Bias, only 2 of the original Halo rings remained intact and operable; while five other more efficient ones had been constructed in secret at the Lesser Ark by Faber.

Each Halo installation was given a designation number, from 01 to 09 and from 11 to 18, and was overseen by a Monitor. The Monitors are given control of the Installations' Sentinels, Constructors and Enforcers, and are responsible for containing the Flood test subjects in their research facilities and protecting the Halo from intruders.

The Array also encompasses installations known as the Arks, Installations 00 and 10 respectively, from which the array can be remotely and safely activated while out of firing range. Inhabitants on the Arks are able to survive the effects. The Forerunners also built active Shield Worlds, Micro Dyson Spheres encased in a slipspace field with their own star in the center and a habitable environment, as a means to escape the effect without having to leave the galaxy.

History
The Halos were constructed by the Forerunners a few hundred years before the Flood were discovered as a communication and transportation portal system, but because their builders saw that their current isolation and containment procedures were not adequate to stop the spread of the Flood, the were redesigned as weapons. 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.

The seven Halos then remained relatively dormant for one hundred thousand years (though at least one of them experienced a major Flood outbreak; and reference given to brief visits by other species by 343 Guilty Spark in "Conversations from the Universe") until one of them, Installation 14 (Alpha Halo), was discovered in 1984 by the theocratic alien superpower, the Covenant. The Covenant, who revered the Forerunner as gods, believed that the Halo's main weapon was actually a source of "Divine Wind" meant to propel them on a path they called the "Great Journey". Installation 07 was discovered by the friedens during first contact, and were used by sovereign to usher the return of the reapers.