Cerberus was an artificial object in orbit of the neutron star Hades that encompassed layered portions of technology from at least three distinct alien organisations. Initially a portal created by the Hades matrix to allow ingress, it was detected by the Inhibitors sometime before the extinction of the Amarantin, and its purpose befouled. They placed a device intended to exploit the portal's intriguing nature to detect the emergence of sentient life.
The Banished, a sub-species of the Amarantin, eventually discovered Cerberus (though it did not yet appear as a moon), and triggered the Inhibitors device. Later -- whether before or after the Inhibitor war is unknown -- the Banished subsumed the two alien creations in layers of massive machinery, covered with a veneer of matter made to appear as a moon. Their hope was to spare other species the fate that had befallen them. Ironically, it would be an agent of their own descendants that would seek to circumvent their safeguards and activate the Inhibitor device.
The outermost layer of Cerberus is camouflaged to resemble an airless, rocky moon. This layer contains hidden projectile launchers and nanotechnological defenses to counteract attempts to penetrate the surface.
Below the outer layer is a hollow space consisting of pillar-like supports and numerous serpentine mechanisms, which can extend through openings in the outer layer to grapple objects and bring them below the surface.
The layer beneath this is composed of massive, tightly packed translucent crystalline objects, which have spaces between them barely large enough to admit human-sized beings.