Onyx

Onyx was a Forerunner shield world located in the Zeta Doradus system. Although initially assumed to be a natural planet by human explorers, Onyx was promptly classified by the Office of Naval Intelligence shortly after its discovery due to its various anomalous features as well as the presence of alien ruins there. An ONI research facility was subsequently established on the world to study the alien artifacts. Onyx served as a training ground for the SPARTAN-III program during the Human-Covenant War until its surface dissolved in November 2552, revealing its nature as an artificial structure. The Amalgam Sentinels that comprised the bulk of Onyx's underlying latticework remained at the site to guard the structure, promptly responding to any incursion within several light-minutes. Though Onyx itself could not be approached, the UNSC managed to establish a listening post at one of the system's outlying planets.

Overview
Onyx exhibits several differences to other shield worlds discovered in the modern era, leading experts to suspect it may have held a special importance to its creators. Externally camouflaged to appear as a natural planet, Onyx concealed a concentric internal structure, buried at the center of which was an antechamber housing a portal to a slipspace-based Dyson shell megastructure, one much larger than any other Forerunner construct. The information regarding Onyx's internal Dyson construct comes from the survivors of the Onyx Conflict, who, thought dead since 2552, unexpectedly returned from the structure in 2559 through a secondary egress portal.

Based on available data, it is strongly suspected that while the external planetary structure and slipspace gateway were of Forerunner origin, the Dyson sphere was not; the Forerunners merely found it, and put it to use as a bomb shelter from the Halo Array. The original builders of the Dyson shell had likely passed into history millions if not hundreds of millions of years before the Forerunners' time. Not only is the structure's size anomalous, but its anchoring in slipspace demonstrates highly exotic dimensional manipulation capabilities. From what can be gleaned, it appears that the Dyson sphere is not physically located in the core of Onyx, at least not in any meaningful sense. Instead, the sphere is a hyperspatial construct, built in a layer of reality far beyond our own. It is effectively everywhere and nowhere at once, locked away beyond layers of slipspace so deep even the Forerunners had trouble accessing them. Onyx's core was just an access point, and those trapped within experienced the transit as instant because there was still no distance involved in the conventional sense.

Although of unknown origin, considerable Forerunner engineering was present in the internal Dyson shell. The refugia habitats appear to have been of Forerunner origin, as well as the internal transit system, all built on an inert shell of exotic matter.

A significant portion of Onyx's outer superstructure was made up of multipurpose drones known as Amalgam Sentinels, which also acted as the world's principal defensive constructs. These are a type of general-purpose machine introduced in the desperate final decades of the Diluvial War, constructed from in-situ resources in the Zeta Doradus system. The shell was to be landscaped to look like a naturally-occurring planet, though this process would continue well after the firing of the Halos. It has been theorized that other entry points into the Dyson sphere may exist on worlds similar to Onyx across the galaxy, though no such worlds have been found.