Onyx Sentinel

The Onyx Sentinel is a deadly and versatile variant of Forerunner Sentinel capable of adapting to different threats and fulfilling a variety of roles both offensive and passive. This variety has been encountered only on the artificial world Onyx, which was actually comprised of trillions of the machines.

Description
The Onyx Sentinel shares a vaguely similar appearance to a monitor—a round metallic sphere a quarter meter in diameter, with a central red "eye" that glows golden. The interior of the sphere, only visible if the casing is breached, glows blue-white with heat. 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 can change position for various purposes, and will rapidly "pop" into place to form a triangular plane when the sentinel's shields are active. The central eye is the weaponry and offensive capability of the sentinel and is capable of unleashing a devastating beam of energy. The booms are covered in a matte silver metal engraved with a subtle circle pattern, while the central eye is surrounded by tiny Forerunner glyphs floating a centimeter off the surface. The central sphere is known to emit a blackbody radiation equivalent to 6,000 degrees Kelvin in its normal active state, and 15,000 degrees Kelvin when charging.

The Onyx Sentinel is also equipped with anti-gravity technology, similar to that of the monitors. Onyx Sentinels can even reach escape velocity and fly into outer space; in ground operations, they normally work a "higher ground" battle mentality. They are able to attenuate their communications systems to various signals, such as those of UNSC COM channels, and are equipped with Forerunner translation technology. During the Battle of Onyx, one attempted to communicate with Ash-G099 using the human root language of Latin, supposedly based on a minimal linguistic analysis of the humans' communication. The Onyx Sentinels are highly adaptive in their behavior, being able to learn from their opponents with each encounter and adjust their tactics accordingly.

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 is resistant to nearly all projectiles, even the rockets of the M41 rocket launcher and Lotus anti-tank 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. 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.

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 impact with explosive power and 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. When they first appeared, a group of Onyx Sentinels destroyed a large mesa on Onyx; Ash-G099 estimated the explosion to be equivalent to a megaton of TNT.

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 in range. This makes them highly dangerous on the battlefield, as a strategy that managed to destroy a Sentinel once will almost certainly not work twice. While they initially operated on their own, the Sentinels on Onyx soon configured into trios with a combined pair of them on patrol and one two or three kilometers above on overwatch. Even the hunter-killed pairs preferred to operate at a high altitude unless forced to descend near ground level.

Onyx Sentinels can 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. Each combination adds cumulative power to their weapons, thrust, and shields. In space, large groups of Onyx Sentinels form crystalline geometries with seven as a baseline: at one point they formed into a triangular lattice of 49 Sentinels consisting of seven line-shaped formations each consisting of seven drones. Such a configuration was easily capable of overpowering a Covenant destroyer. When engaging Voro Nar 'Mantakree's fleet, thousands of the Sentinels combined into elaborate octahedral geometries. Although they do not possess a slipspace capability on their own, the combined Sentinel formations are capable of tapping into other vessels' slipspace drives and using the active slipspace matrix to perform short-range jumps to gain an edge in battle. The space-borne Sentinel formations were also capable of effectively capturing the energy of Covenant plasma torpedoes and rendering them harmless.

Non-combat roles
Onyx Sentinels are also known to have other specific tasks beside combat, much like other types of Sentinels. Because of their modular design, 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 in Onyx's Zone 67 in order 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, arranged into elaborate scaffolding resembling the carbon bonds of a polymer chain. This latticework made use of the Sentinels' components in a way that the central spheres served as joints, connected to one another by the cylindrical spars. In the end of the Battle of Onyx, as the portal to the shield world closed, the artificial planet dissolved with the sentinels obliterating a Covenant fleet.

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.

List of appearances

 * Halo: Ghosts of Onyx
 * Halo: Glasslands
 * Halo Mythos: A Guide to the Story of Halo