An oculus is a mechanical servitor of the Collectors, apparently built with a single purpose -- to find and eliminate threats. Its outer structure has three apertures that house long-range detection equipment similar to those found on deep-space probes, used to find and track targets from near-stellar distances. Contragrav mass effect fields allow an oculus to move easily through space or atmosphere in pursuit of its prey, and a large central lens serves as the emission center for its weapons platform. All four of its lenses have a titanium iris covering that prevents the detector's optics from scratching due to interstellar dust or atmospheric residue.
Upon its destruction, more of the oculus' features become apparent. Its interior houses fibers of near-inorganic nerves, indicating that a Collector drone has been stripped down to its central nervous system, reinforced with interfacial gel that can withstand hard vacuum, and repurposed to pilot the oculus for the rest of its existence.
The central weapon is a strange one, operating on alien principles. It appears to use mass effect fields and directed energy to attack the strong nuclear force that holds its targets' atoms together, producing heat and light in an air atmosphere and causing irradiating gamma particles to burst from any heavy metals targeted by the beam. Such a weapon could easily cut through shields, and leave its target dying with just a glancing hit.