Onyx Sentinel

The 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 used on other similar worlds is currently unknown.

Description
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 is also equipped with an anti-gravity device, 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 Sentinels also share an attribute of a "dumb" AI in that an they can —within its respective parameters— learn from other beings, being able to learn from their opponents with just one encounter.

Combat
The Onyx Sentinel is one of the deadliest sentinel variants encountered to date, and is a powerful combatant on both the ground and in space. Unlike the shields of ordinary sentinels, which can be disrupted by conventional gunfire, Onyx Sentinels are protected by a spherical gold-colored energy shield that resistant to nearly all projectiles, even the rockets of the SPNKr Launcher and LOTUS Mine detonations. However, whereas normal sentinel shields are constantly in a state of activation, the shields of an Onyx Sentinel only activate when the machine detects the approach of fast-moving objects, such as bullets or plasma bolts. While the reasons for this selective shield use are unknown, this behavior does provide an exploitable advantage, as the Sentinel can be duped by slow-moving projectiles such as falling rocks. In addition, an individual Sentinel cannot fire its weapon and activate its shields at the same time.

In combat, Onyx Sentinels possess adaptive behavior that is unique among known Forerunner constructs. The Sentinels are linked to one another, with each Sentinel recording and transmitting combat footage to one another. As they process new information, they adjust their strategies and combat behavior accordingly, and broadcast the altered battle plans to all Sentinels on the planet. This makes them highly dangerous on the battlefield, as a strategy that managed to destroy a Sentinel once will almost certainly not work twice.

Onyx Sentinels can also merge together, re-arranging their central spheres and orbiting booms into new, combined forms. These configurations can serve a variety of purposes; a pair of Sentinels can combine so that one can project a shield while the other fires, while a large group of Sentinels can form a cluster that can fire a single, devastatingly powerful beam of energy.

Onyx Sentinels fire golden, lethal beams of energy from their central eye, which are thought to be similar in strength to the beam weapon used by Monitors. These beams are powerful enough to blast apart rock formations and vaporize organic targets on contact. When multiple Sentinels merge to combine their fire, they can unleash a beam capable of destroying large starships.

Other roles
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.

To maintain the Sentinel population of Onyx, the artificial planet had a massive Sentinel production facility that produced a new Onyx Sentinel every six seconds.