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Onyx or XF-063 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 a Shield World. It was the location of a large battle, the Battle of Onyx.

Background
Onyx has an Oxygen-Nitrogen gas atmosphere, of suitable and apposite pressure for Human life with a temperate weather cycle. It also has a shocking abundance of Indigenous Flora and Fauna, some of which are edible beings and pose no threatening danger. The transplanting of Earth species was possible.

Some notable anomalies were that it had no detectable tectonic activity, an unusually strong planetary magnetic field, and slightly higher than normal background radiation.

History
Onyx was discovered by the UNSC Engineering Corps sometime around May, 2491. In ENGCORP Field Report A76344, Captain D. F. Lambert said "XF-063 is a rare jewel of a find" and recommended colonization.

On February 19, 2492, in ENGCORP Field Report A79052 Lieutenant W. K. Davidson stated that Geological Expedition Four found and entire phylum of possible alien taxonomy and recommended a follow-up survey with specialists in Paleo, Xenobiology and Biochemistry.

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 no results. 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 (there were three companies trained). 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 X-Ray was rumored to have encountered a "glowing eye," which detonated upon discovery.

Battle of Onyx
In 2552, Onyx's Sentinels were roused to action by the standby signal from Installation 04, instigating the Battle of Onyx and revealing Onyx's true nature. The AI Endless Summer took command of all military personnel on Onyx to immediately defend Zone 67 per General Order 98.93.120, after Onyx's Sentinels first started to attack the SPARTAN-III's. Endless Summer sent Kurt a flash transmission sending attached files directly into Kurt's retina. An explosion stopped Endless Summer's transmission before Kurt could finish scanning the new threat.

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 the Shield World.

Locations on Onyx
Here are a few important places on and around Oynx.


 * Camp Currahee
 * Command Center
 * Armory
 * NCO quarters
 * NCO's Inspection yard
 * Infirmary
 * Camp Commandant's residence.
 * The Tree House
 * Zone 67
 * Slipspace COM Launcher
 * Twin Forks River
 * Gregor Canyon
 * El Morro Point


 * Shield World
 * Core Room Antechamber
 * Sentinel Production Facility
 * Natural Satellite (Onyx's Moon)

Trivia

 * It is rumored that the planet in the Halo 3 Legendary Ending is Onyx. However, this is contradicting the fact that Onyx was destroyed into oblivion, leaving behind a Shielded Dyson Sphere guarded by Onyx Sentinels. Another possibility is that it introduces Marathon.
 * The Terminals mention that there were indeed Forerunners on Onyx. However, the Terminals were probably referring to The Onyx Sentinels being on Onyx.
 * The Sentiniels on Onyx were much stronger than the ones on Installation 04 and Installation 05, being able to combine, and were space worthy.
 * The Covenant Translating Oracle has no definition for the word "onyx".