The Onyx Conflict was a major engagement between UNSC, Covenant, and Forerunner forces on and around the planet of Onyx in the Zeta Doradus system in October and November 2552. The conflict saw major skirmishes between UNSC and Covenant forces, as well as between Forerunner and UNSC forces. The battle resulted in the de-coalescence of the planet Onyx into a swarm of trillions of Forerunner Sentinels, and the discovery of the Sarcophagus by Spartan-II Blue Team and several Spartan-IIIs including Tom-B292 and Lucy-B091. Additionally, Battle Group Omicron, sent to intercept the Covenant forces headed to Onyx, was wiped out by the Covenant fleets in orbit.
The conflict on Onyx was initiated by the destruction of Installation 04 by Master Chief Petty Officer John-117. The destruction of that Halo emitted a signal to all other Forerunner installations, and placed them in standby mode. This woke up the sentinels on Onyx which began attacking the UNSC forces stationed on the planet.
The battle on the ground was significant, leading the UNSC forces to discover and enter the entrance to Shield World 006, a Forerunner shield world the size of Earth's orbit around the sun, but hidden in a slipspace bubble 23 centimeters in diameter. During the conflict on the ground, several Spartans were killed, including Will-043, Kurt-051, Dante-G188, and Holly-G003.
These events are depicted in the novel Halo: Ghosts of Onyx.