From Halopedia, the Halo wiki
Sub-Sector 35 is a largely unexplored and presumably uninhabited sector of space outside the UNSC's sphere of influence. The sector is the location of an unidentified moon which houses the Forerunner Line Installation 1-4. This makes it a dangerous area to traverse, even in Slipspace, as the Installation's defense systems would destroy any ships passing nearby.
In ancient history, the Forerunners built the Line Installation to the sector as a defense weapon and a Flood research facility. Over the course of history, dozens of ships passing through the sector had crash-landed on the moon, leaving its surface littered with wreckage.
During the Human-Covenant War in 2552, this sector was used as a primary supply route by the Covenant. ONI Section III sent Spartan Team Black, secretly onboard the Long Time Coming, to board and destroy any Covenant vessels in the area. Meanwhile, the Covenant CAS-class assault carrier Clarity of Faith was to rendezvous with a supply convoy to escort them through the sector.
Prior to jump into Slipspace, both ships received unusual burst transmissions with a Pi-constant baseline. The messages were unclear, seemingly depicting various celestial objects, but were apparently warning of a significant astrogation hazard in the sector. While in Slipspace, both the human and the Covenant ship were shot down by the Line Installation's energy weapon. They dropped out of Slipspace and crash-landed on the unknown moon.
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