From Halopedia, the Halo wiki
Ephsu is a star system in the Sagittarius Arm of the Milky Way, notable as the location of Installation 07. It is an otherwise unremarkable M-dwarf star system, with one, uninhabited terrestrial planet, which serves as the anchor for the Halo.
The system was discovered as Installation 07's location by renowned human scientist and historian Doctor Luther Mann after a year of sifting through data he'd discovered during his stay at the ONI Research Facility Trevelyan. There was a research outpost on the Halo established by March 2555, which was protected by at least two Halberd-class destroyers.
During the Created conflict in 2559, the system was likely the site of a major battle. The Banished, led by War Chief Escharum, invaded the Halo. A battle both in space and on the installation took place, with the UNSC suffering major losses, leaving the installation occupied by the Banished, and badly damaged.
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