From Halopedia, the Halo wiki
This unidentified moon is a natural satellite in a remote planetary system. Located well beyond humanity's Outer Colonies, it was occupied by the Covenant during the Human-Covenant War. The moon was apparently home to Forerunner artifacts, as the UNSC believed it to be one of the Covenant's religious dig sites. Two teams of SPARTAN-III Headhunters were deployed on the moon to destroy several Covenant base camps before the UNSC forces would begin a full-scale assault. Eventually, both teams were eliminated by the Covenant forces.
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