basically just a flying ring lol

"I discovered the Ark keeps an almost finished ring in its central forges at all times. It's an emergency replacement in case one of the other rings breaks down."
— Ellen Anders on the Halo

In the final months of the Human-Covenant War, Installation 00 constructed a replacement for Installation 04, which had been destroyed by John-117 during the Battle of Installation 04. The still-incomplete replacement ring, Installation 08, was launched from the Ark's Foundry during the war's final battle, but was subsequently destroyed by its own premature firing, initiated by the Master Chief and Cortana in order to sterilize a Flood incursion on the Ark. The ring's firing severely damaged the Ark, though the massive installation was able to repair itself in time, and by March 2559, the Ark's Foundry had finished constructing a new replacement ring, Installation 09.

During the battle between the UNSC forces of the UNSC Spirit of Fire and the Banished occupying the Ark, the newly built replacement Halo was discovered by Professor Ellen Anders. Consequently, she and Captain James Cutter formulated a plan to deploy the ring to the Soell system, Installation 04's former site, where it could be used to raise a distress beacon to the UNSC.[3] The plan was not approved until Isabel proposed a plan to destroy Banished's assault carrier Enduring Conviction when it launched an assault on UNSC Spirit of Fire after Decimus was defeated[3]. After succeeding in destroying the flagship, Anders went to the Foundry and activated the installation, prompting Atriox to order all available units to take the ring from the UNSC. While the Banished contested the UNSC for control of the new Halo, the humans gained a foothold, allowing Anders to enter the control room where she successfully disarmed the ring's firing mechanism and manipulated its gravity anchors to eject a section of the Halo's landmass housing Banished forces into space. Although the UNSC forces evacuated to their ship, Anders could not make it out of the control room in time before Installation 09 entered a slipspace portal to the Soell system. En route to its destination, however, the Halo was intercepted by a Guardian.