From Halopedia, the Halo wiki
The Battle for Zeta Halo is an ongoing conflict between the UNSC and Banished on Installation 07, the oldest surviving ring of the Halo Array. It began in late 2559 when the UNSC Infinity arrived at the installation with the goal of capturing and destroying Cortana. However, upon exiting slipspace, they were ambushed by a Banished force led by Atriox himself. The UNSC was defeated and the survivors were marooned on the Halo, struggling to survive for months afterward. That is, until Spartan-II John-117, the legendary Master Chief, was found alive amidst the debris of the initial space battle. With his return, the UNSC forces on Zeta Halo have discovered hope once more.
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