Onyx was an artificial, habitable planet in the Zeta Doradus system that was primarily used for training all Spartan-III recruits from 2531 until its destruction in 2552. The planet was discovered in 2491 and was found to have an atmosphere suitable for human life, which made it an ideal candidate for colonization. When Forerunner artifacts were discovered on the planet, however, the Office of Naval Intelligence removed the planet from navigational records, and began studying the artifacts in earnest.
Upon the onset of the Human-Covenant War, however, research efforts on Onyx were reduced drastically, having thus far yielded little to no results. In 2531, the planet became the site of the Spartan-III training program, being that there was already an ONI presence on the surface and the location was a secret. Three companies of Spartan-IIIs were trained on Onyx by Franklin Mendez and Kurt-051, all with a much higher success rate than that of the Spartan-II program.
Following the Battle of Installation 05 and the placing of Delta Halo into standby, the Forerunner sentinels on Onyx were awoken, causing major destruction to most military forces in the area. A few remaining forces on the ground, including Kurt, Mendez, Doctor Catherine Halsey, part of Gamma Company, and Blue Team, managed to get to the planet’s core, where they discovered a slipspace rift that allowed them to travel to the Sarcophagus, a Dyson sphere shield-world contained within a slipspace bubble only twenty-three centimeters in diameter. Immediately following their entry into the slipspace rift, Onyx fully decoalesced into a swarm of trillions of sentinels, leaving only the slipspace bubble containing the Dyson sphere and handful of UNSC forces.