Onyx, officially known by the UNSC as "XF-063", was a planet-sized, Forerunner construct located in the Zeta Doradus System. While it had a Human habitable surface and a seemingly terrestrial crust, internally, it was actually composed of trillions of Sentinels. Onyx's ultimate purpose was to guard the entrance to a Shield World. It was the location of the massive, three-sided Battle of Onyx.

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Onyx