Onyx

Onyx was a large, 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 Shield World.

Onyx was discovered by the United Nations Space Command Geophysics Corps in 2491. They thought it was a planet and named it after the large deposits of onyx rock. They noted that it was already capable of sustaining Earth life, despite the elevated levels of background radiation, and was already home to many indigenous forms of plant and insect life.

In 2511, when archaeological expeditions uncovered massive subterranean Forerunner constructions in Zone 67, ONI immediately took control of the world, going so far as to remove Onyx from UNSC navigational databases and suspending any prospective colonization and terraforming attempts. However, further attempts to determine the purpose of the structures, or even to extract any usable technology, proved fruitless. In 2525, the operation was scaled down and only a minimum ONI garrison and a single AI remained. The planet's location remained secret. Regular scans continued to yield little of value.

With the onset of the Human-Covenant War, Vice Admiral Margaret O. Parangosky authorized the construction of the SPARTAN-III training base on Onyx in 2531. This base, designated Camp Currahee, was constructed perilously close to Zone 67, which had been declared strictly off limits. However, during a routine exercise north of Currahee, Beta Company Team X-ray trespassed into Zone 67, entered a Forerunner structure, and disappeared without trace. An ONI investigation into the matter was inconclusive, although in hindsight it was likely Team X-ray were either captured or killed by Sentinels.

In 2552, Onyx's Sentinels were roused by the standby signal from Delta Halo, instigating the Battle of Onyx and revealing Onyx's true nature. At the conclusion of the battle, Onyx was effectively destroyed, although not before Bloody Arrow was declared and Blue Team, Team Saber, Team Katana, Dr. Halsey, and SCPO Mendez escaped to the safety of Shield World.

Notable locations on Onyx

 * Camp Currahee
 * Zone 67
 * Core Room Antechamber
 * Shield World
 * Slipspace COM Launcher
 * Sentinel Production Facility