Soell

The Soell system, classically known as Iota Horologii, is a strategically-important star system of nine planets located in the Enuma Sector of the Orion-Eridanus Quadrant.

In the waning years of the Diluvial War, it was chosen as the site where Halo Installation 04 would be deployed, which was activated with the other Halos to wipe out all sentient life in the Milky Way galaxy. It was discovered by the crew of the in September 2552, who were pursued from Reach by the Fleet of Particular Justice. In the ensuring battle, Alpha Halo was destroyed by John-117.

In the aftermath, the Covenant established a large presence in the system in order to research the various relics that survived the Halo's detonation. The UNSC and the Concord of Reconciliation captured the system some time after the Human-Covenant War. Under the Treaty of Arusha, the Soell system is designated as a Forerunner site of strategic importance, and considered a formal Joint Occupation Zone with a permanent military and scientific presence by both parties.

Threshold
A super-Jovian gas giant, Threshold houses an array of atmospheric gas extraction and refinery facilities built centuries before the Halos by a far-flung expedition by the Miner rate. Dozens of individual gas mining stations hang by suspension cables from an enormous high-altitude refinery structure, with a handful refitted as Flood research stations during the Diluvial War. Though many have been long decommissioned, some of these facilities remain active, with automated systems still fulfilling their ancient directives. Outside drone mapping and a handful of cursory manned missions, many of these facilities have still not been explored. However, Concord scientists have devised a means to utilize the operational refineries to supply the local joint fleet with fusion fuels, eliminating the need for new fuel-harvesting infrastructure.

One of the gas mining facilities, Extraction Platform 02-17, was explored by a Covenant extreme-conditions artifact retrieval team which subsequently turned heretic after contact with 343 Guilty Spark and the monitor's revelations as to the nature of the Halos. The facility was plunged into the depths of Threshold by the recently-appointed Arbiter, Thel 'Vadam, to prevent the spread of the Flood which had broken loose on the station.

Halo Installation 04
Due to potential continued Flood contamination, the debris field remains an exclusion zone enforced by the UNSC and Concord fleets patrolling the area. Scientific incursions continue to be made to the segments of the broken ring as well as various other Forerunner facilities around Threshold, though precautions are extreme. By the 2570s, these precautions had been loosened somewhat to allow pilgrims with permission to land under certain conditions.

Basis
A large natural satellite of Threshold, Basis housed a small heretic camp that went undetected until nearly a year later. Much debris from Installation 04 rained down on Basis, making any surface incursions hazardous not only because of the debris itself but potential Flood that may have survived within it.

Vector
One of Threshold's smaller and further-out moons, Vector houses a secondary Forerunner facility first used by the Ministry of Tranquility artifact retrieval teams of the Fleet of Infinite Wonder. The outpost was later repurposed as an impromptu base by the UNSC and the Schismatics after the system's capture in 2553. Subsequently named Keyes Junction in honor of the late Jacob Keyes, the facility saw significant expansion in the post-war era, becoming the hub of the scientific and military efforts in the system.