Trevelyan

Were you looking for Onyx, the material?

"XF-063 is a rare jewel of a find."

- Captain D. F. Lambert

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.

Background
So named by the United Earth Government due to the unusually large onyx quarries found in the planet's crust, Onyx had an Oxygen-Nitrogen gas atmosphere, of suitable pressure for Human life with a temperate weather cycle. It also had an unusually large number of indigenous species of flora and fauna, some of which were edible by Humans and posed 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. All of this may possibly be explained by the presence of large numbers of Sentinels beneath the planet's surface.

Structure
Onyx's surface was composed of seemingly infinite amounts of Sentinels, with a massive Forerunner city hidden under the terraformed ground layer. Under the surface, there was a vast network of underground tunnels and installations, including a map room and an immense Sentinel production facility under the northern polar region.

In the core of Onyx, there was a large room, the Core Room Antechamber. It contained a Slipspace rift that would be open for a short time after the Halo Array was put into standby mode, allowing evacuation to the Micro Dyson Sphere within.

In the center of Onyx resides a Slipspace bubble containing a massive Dyson Sphere habitat, which in normal space, is only some meters in diameter, and inside the Slipspace bubble, has a diameter of approximately 150 million kilometers. The megastructure has a habitable, terraformed surface and in the center, a G2-type star.

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 Paleobiology, 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, yielded 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, Alpha, Beta, and Gamma). 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 05, 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

 * 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.&#160;John-117 drifts toward the planet and&#160;the game&#160;shows an explosion on its surface. According to&#160;the book: Halo: Ghosts of Onyx, Blue Team, Team Saber, Team Katana,&#160;Halsey, and Mendez all made it into the safety of the Dyson sphere save for Spartan Kurt, who stayed to detonate a nuclear device thereby awakening&#160;the sentinels composing&#160;Onyx.&#160;The planet&#160;was then&#160;ripped apart, leaving behind only the Slipspace rift to the Micro Dyson Sphere, guarded by the&#160;trillions of Onyx Sentinels. The explosion witnessed in the game may be that nuclear detonation right before the sentinels emerge. This is unlikely since Zeta Doradus is just outside UNSC space, but Cortana thinks it will take years for someone to find their beacon.
 * The Terminals mention that there were indeed Forerunners on Onyx. However, the Terminals were probably referring to the Onyx Sentinels being on Onyx or the construction of it to become a Shield World to be protected from any of the the Halos.
 * The Sentinels on Onyx were much stronger than the ones on Installation 04 and Installation 05, being able to combine forms, and were capable of space flight.