An Onyx Sentinel is a Forerunner-created machine designed to protect and maintain the Forerunner artificial world known as Onyx. The Sentinels were a more deadly and versatile variant of the standard Sentinel design found on other Forerunner installations. This variety has been seen only on the artificial planet Onyx, which was actually comprised of trillions of the machines. Whether the same model is on other similar worlds is currently unknown.
The Onyx Sentinel shares a vaguely similar appearance to a Monitor - a round metallic sphere a quarter meter in diameter, with a central "eye" that glows golden. Hovering around this sphere are three meter-long cylindrical spars or "booms," which are suspended in midair at equidistant points around the sphere. These booms could change position for various purposes, and would rapidly "pop" into place to form a triangular plane when the sentinel's shields were active. The central eye is the weaponry and offensive capability of the sentinel as it charges and unleashes what appears to be a devastating array of energy.
The Onyx Sentinel also has anti-gravity technology, similar to that of the Monitors. The Onyx Sentinel can even reach escape velocity and fly into outer space; in ground operations, it normally engages its targets at a height of three meters, working a "Higher Ground" battle mentality. It is also able to attenuate its booms to the various signals, such as that of UNSC COM channels. The sentinels are also equipped with Forerunner translation technology and can actually speak the Human archaic language and root language of Latin.
The Onyx Sentinel is considered to be the most dangerous classification of any known Sentinel, is extremely versatile, and a powerful combatant on both the ground and in space. One of their primary advantages is a gold-colored Energy Shield that is entirely resistant to all projectiles, even the rockets of the SPNKr Launcher and LOTUS Mine detonations; however, the motion sensors that activate the shield are duped by slow moving objects, such as a rock, which was used to destroy one Sentinel on Onyx. It is unknown why the shields only snap into place when fast moving projectiles are coming in the direction of the Sentinel (it is unlikely that they do so to conserve energy, for the enormous output of energy per beam is, presumably, far greater than the amount used to hold shields in place), but it's possible that it has flaws in its sensors, and objects moving at slow enough speeds can be virtually invisible to it.
It is also very adaptive, each Sentinel transmitting its combat footage to an unknown intelligence that continually upgrades Onyx Sentinel combat algorithms. As per a Spartan: "We are teaching it how to fight." The Onyx Sentinels can also merge for a variety of purposes, such as combining to increase firepower and shield strength, architectural purposes, and forming clusters to protect Onyx. Even a pair of Sentinels is exponentially more dangerous than a lone one, as a pair, unlike individual Sentinels, can activate its shields and fire its energy weapon at the same time.
It is far superior to the regular Sentinels that guard the Halo installations and The Ark and even the Sentinel Majors of Installation 05 due to its shielding and extreme firepower. Onyx Sentinels fire golden, lethal beams of energy from a central eye, which is thought to be similar in strength to the beam weapon fired by 343 Guilty Spark at several points of Halo 3 when the player is on the Ark, not just fighting him at the end. This attack causes devastating damage: the destruction of a granite plateau, vaporization of a combat bunker, and the instantaneous destruction of any ground vehicle or infantry, even including a SPARTAN-III. Enough Onyx Sentinels (about forty-five) can even vaporize a large starship with apparent ease.
However, they can still be defeated; the Spartans on Onyx once destroyed one at extreme altitude with multiple rockets and sniper rounds, ultimately depleting its shields and destroying it. Although ground troops with only small-arms are virtually helpless when they form larger groups, once these formations are separated they become easy targets for units with heavier firepower; for example, Fleet Master Voro Nar 'Mantakree's fleet dispatched to Onyx destroyed many Sentinels after they were forced out of formation.
Other Roles Edit
Onyx Sentinels are also known to have other specific tasks beside combat, much like other types of Sentinels. Because of their modular shape, they can transform into several different types of configurations. For example, they can interlace into oblong-shaped formations, which are used for excavating and earth-moving. In the center of the worm-shaped configuration travels a continuous stream of stone. This type of configuration has been observed excavating the underground Forerunner city on Onyx, to allow access to the Shield World within. These variants are not hostile, unless provoked.
Another, more original role of the Onyx Sentinels was making up the entire planetary structure of Onyx. Under the artificial and entirely terraformed surface were trillions of these machines, forming the megastructure. In the end of the battle of Onyx, as the portal to the Shield World closed, the artificial planet dissolved with the sentinels decimating a Covenant fleet. It is unknown if they rearranged to form the planetary structure after this, this is unlikely as the shield world was revealed from the destruction of Onyx.