Warning! This post contains SPOILERS for Halo Season 2 episode 8, "Halo"

Summary

• The Halo Rings were built by the ancient Forerunners as a last-resort weapon against the Flood.
• The rings are activated by Reclaimers to wipe out all lifeforms as well as to contain Flood spore samples.
• There are seven Halo installations in the galaxy and one beyond it, with only one Halo Array firing event recorded in the core canon's ancient history.

The titular rings from the Halo franchise are highly coveted by humanity and the alien force known as the Covenant. However, the powerful installations and their history date much further than the Human-Covenant War seen in the canon of the games and books. Likewise, the same is true for the live-action Halo series streaming exclusively on Paramount+.

Set in its own Fall of Reach. It's also revealed the key artifacts pointing to the powerful rings built by those who predate both races by millennia, and the rare few who can access the technology and have been called to seek the rings out. To that end, here's the history and origins of the Halo rings in the core canon, who created them, and what they actually do.

The Halo Rings Were Built By The Forerunners

An Ancient Race of Powerful Beings

100,000 years before the events of the Human-Covenant War, the Halo rings were constructed by the ancient beings known as the Forerunners. A highly advanced technological species, the Forerunners believed themselves to be the protectors of the galaxy, tasked with guarding all known life. This was especially true in the face of the extra-galactic parasites known as the Flood.

The Forerunners commissioned the construction of the Halos, ring-shaped artificial worlds capable of ending the Flood scourge once and for all.

After 300 years of fighting the Flood who began overtaking multitudes across the galaxy, the Forerunners had exhausted all of their technology and strategies in their effort to contain and wipe out the parasitic species for good. To that end, the Forerunners commissioned the construction of the Halos, ring-shaped artificial worlds capable of ending the Flood scourge once and for all. To that end, the origins of the Halos look to be the same in the Halo show as well (despite its unique timeline).

What Do The Halo Rings Actually Do?

A Worst-Case Scenario For The Flood

Essentially, the Halo rings have two functions in both timelines. First and foremost, each ring is capable of being activated by a Reclaimer, of a race chosen to inherit the Forerunner's galactic Mantle of Responsibility (in this case humanity). Once activated, a Halo ring is capable of firing a devastating energy pulse designed to wipe out all lifeforms within a maximum effective radius of 25,000 light-years. To that end, the goal is to effectively starve the Flood by taking away its food source, a weapon of last resort in the event of the parasites completely overwhelming the entire galaxy.

The second purpose of the Halos was to act as research facilities for the study and containment of various Flood samples. Also known as Installations, the Halos were largely maintained by Forerunner A.I Monitors, meant to assist Reclaimers in activating the rings in the event of a Flood outbreak. As such, it's understandable why the devastating power of a Halo ring would be so coveted by the Covenant in their mission to usurp humanity's chosen role as the Forerunners' successors by wiping them out (and why humanity wants to keep the rings out of the alien alliance's hands).

How Many Halo Rings Are There?

Installation 04 Is One of Many

While there were originally 12 Halo rings constructed by the Forerunners, several were destroyed and rebuilt in the ancient past, leaving seven total Halo installations across the galaxy numbered 01-07. There was also Installation 00, located outside the known galaxy which could be accessed via portal technology. From Installation 00, the entire Halo Array could be activated remotely and all at once, ensuring that the entire galaxy could be purged with one single action.

The Halo Array Has Only Been Fired Once

The Flood's First Defeat

In the core Halo canon, the entire Halo Array has only been fired one single time shortly after its construction. Having no other options to stop the growing Flood, the Halo Array was activated by the Forerunners, a devastating act that saw the advanced race sacrificing themselves for the sake of the galaxy's future. As such, the Halo Array brought The Flood to the very brink of extinction.

That said, the Flood managed to survive as seen in the very first Halo game (Combat Evolved) when the Covenant and UNSC inadvertently unleashed spore samples of the parasitic race from their containment on Installation 04, the first discovered ring in the present era. However, Halo season 2 saw the Flood making their resurgence after Miranda Keyes and her mother Catherine Halsey discovered and removed a spore sample that had been locked away in a Forerunner laboratory located on the planet Onyx.

The Forerunners' Plans After Using The Halo Array Explained

Preserving Worlds and Reseeding Life

The only way exterminating all sentient life to save the galaxy makes sense is if there was also a plan in place by the Forerunners to ensure life could be reborn following the Flood's extermination. The Forerunners gathered extensive DNA samples so that life could be "reseeded" after the rings did their work, and various planets known as shield worlds were prepared to protect them from the rings' power. Onyx was one of these shield worlds, its status having been teased in Halo season 2 via the Forerunner ruins as well as the laboratory that was discovered with all of its samples (including the Flood's).

Master Chief's History With The Halo Rings Explained

Installation 04 and Beyond

In the core canon, Master Chief and Cortana first land on Installation 04 (aka Alpha Halo) where they soon learn the Halo rings' true purpose alongside the newly unleashed Flood threat. Using the UNSC ship The Pillar of Autumn, Master Chief destroys Installation 04 in Halo: Combat Evolved, preventing the Halo from ending all sentient life in the known galaxy. That said, Installation 04 would later be rebuilt by Installation 00 in subsequent Halo games, becoming Installation 08 which John-117 and The Arbiter Thel' Vadam use to destroy the Flood's Gravemind and the overrun Installation 05.

Most recently, Installation 07 (aka Zeta Halo) is the primary backdrop for Halo: Infinite, the latest game which pits John-117 and the UNSC against the new Jiralhanae-led force known as the Banished and the Endless, a race who'd been locked away on the Zeta Halo by the Forerunners. However, Halo season 2 has only just brought Master Chief and Cortana to Installation 04. As such, it's an exciting prospect as it means their journey has truly begun, one that will come with its share of surprises given the freedom of its own unique timeline, assuming a third season gets greenlit.

All episodes of Halo season 2 are now streaming exclusively on Paramount+.