Delta Halo, formally Installation 05, was the fifth Halo built for the Neoteric Halo Array. It orbits the gas giant Substance, which is in turn orbiting the star Coelest in the Norma Arm of the Milky Way galaxy. At the tail end of the Forerunner-Flood War in 97,445 BCE, Delta Halo was fired for the only time as part of the Great Purification that cleansed the galaxy of all sentient life and eliminated the Flood threat.
Sometime prior to the halo's discovery by the Covenant, likely thousands of years beforehand, the Flood being studied on the facility escaped containment, establishing a Gravemind and creating a significant threat to all life on the ring and anyone who landed on the surface.
The ring was visited by the Covenant in 2552 during the Battle for Earth, who were attempting to fire the array to begin the Great Journey. UNSC forces followed the Covenant, successfully stopping the firing of the halo, though not before placing the entire array in standby. Following the Battle of Installation 05, the Forerunner construct was left alone, though the UNSC placed a space station in orbit around the halo to keep tabs on its status and study it prior to decommissioning.