Discovered in 2491 CE by the UNSC Engineering Corps, knowledge of its existence was kept under lock and key for decades after its discovery. Of primary interest to the UNSC were the forerunner structures discovered underneath the surface of the planet. Being unsuccessful at determining the true nature of the artifacts, the planet was later converted into a training facility for the Spartan III's.
 Activation and Purpose
When the Halo Installation 05 aborted firing and sent out a standby signal, trillions of Onyx Sentinels were awoken, providing proof that the entire planet was composed of them. As a result of this, Dr. Catherine Halsey and the surviving Spartans from Reach stumbled upon the true meaning of the planet Onyx: a portal to a Shield World. Contained within the depths of the planet was a doorway to what Dr. Halsey determined to be a parallel dimension containing a micro Dyson Sphere. The purpose of Shield Planets such as Onyx was to protect Forerunners from an imminent or planned Halo installation activation.
In 2552CE, when all the Onyx Sentinels were activated, Spartan Kurt-051 detonated a pair of nuclear warheads, destroying a large part of the planet, including the portal to the Shield World.
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