Alpha Halo, formally Installation 04, was one of seven Halos in the Neoteric Halo Array. It was positioned in the Soell system, orbiting the gas giant Threshold, and operated by the station's monitor 343 Guilty Spark. At the tail end of the Forerunner-Flood War in 97,445 BCE, Alpha 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.
The installation remained undiscovered until 2552, when the smart AI Cortana, using data recovered from Forerunner artifacts, plotted a course for the UNSC Pillar of Autumn away from the recently glassed Reach and ended up at Alpha Halo. During the Battle of Installation 04, the Flood was released, forcing John-117 and Cortana to destroy the ring by detonating the Pillar of Autumn's reactors. A piece of Alpha Halo, known as Alpha Shard, made an emergency jump through slipspace and ended up in the Orrichon system, where a particular material was harvested from its surface and used as a bio-weapon before finally being neutralized by an ONI team.