Steward Sentinel

"Stewards: These Sentinels accompany other Forerunner machines and assist them with their tasks, using a combination of support systems and offensive weaponry.The largest constructs are classified as strato-sentinels, taking the roles of grand builders and overseeing architects for major Forerunner works. Those used in military applications were often classified as weapon-ships, capable of being fitted with naval-scale ordnance and defensive systems."

- Jeff Easterling

The Steward is a type of Strato-Sentinel, and are massive terraforming machines, used to gather raw materials for large-scale Forerunner astroengineering projects such as the Halo Array. Retrievers are designed to perform terraforming projects, and are capable of mining moons, asteroids, or other planetary bodies.

Description
"Our records indicate that it employs an artificially produced gravitic force to remove minerals from a planet's surface. Those minerals in turn are used to build Forerunner structures and installations."

- Usze 'Taham, referring to the Retriever Sentinels

Dark gray in color, Retrievers are massive and powerful Sentinels that are slightly smaller than the average human frigate, which are about 478 meters in length. The upper section of a Retriever is vast and wide, while the lower section is lined with numerous mechanical, tentacle-like "legs" that move in pursuit of objects. Glowing eye-like lights are installed on the front of each Sentinel.

A Retriever is capable of producing an artificial gravitic force that allows it to remove materials from a world's surface. Once in position above an acceptable place to harvest, the Retriever deploys its blue, gravitic beam towards the world's surface and begins hauling chunks of land caught in the beam into its underbelly. The mining process utilized by a Retriever is noted to create a tornado-like effect as the ground is pulled into the Sentinel's hold. If deemed necessary, a Retriever is capable of connecting and merging with another Retriever and forming a single unit. Combined Retrievers somewhat resemble an average Retriever, though they seem to be more effective in combat and are significantly more maneuverable. With no apparent drawbacks, a combined Retriever seems to take on the benefits of both Retrievers' speed and weaponry.

As the Sentinel was not designed for combat situations, Retrievers have little to no shielding. In 2555, monitor 000 Tragic Solitude outfitted thousands of Retrievers with massive energy weapons mounted on their fronts. These Retrievers were capable of utilized powerful volleys of energy against opponents and were significantly faster and had greater maneuverability in space. Once destroyed, Retrievers typically exploded into a giant fireball that engulfed its surroundings.

Roles
"I thought Retrievers were just for mining and gathering minerals!"

- Captain Annabelle Richards, upon seeing a heavily armed Retriever

The Retrievers are among the most powerful of the Sentinels employed by the Forerunners, with their primary purpose being the mining, gathering, and transporting of minerals and other natural resources from a world. Using its artificially created gravitic force, Retrievers extract materials from a world's surface and transport to construct Forerunner structures and installations. Retrievers were also used by the Forerunners as escorts to keyships and larger Sentinel variants.

The Forerunners utilized Retrievers to help in the construction of the Halo rings, such as the newly constructed Installation 08 in 2552, and the maintenance of Installation 00. Retrievers mined and gathered raw materials from the planetoid hosted in Installation 00's center and transported them to a forge within the Foundry that refined, processed, and stabilized the materials into the mold of a Halo installation. Although usually automated, Retrievers can be controlled by a monitor. Monitors are capable of effectively utilizing Retrievers in combat, and can have them overwhelm opponents.

A Retriever is programmed to complete its tasks with no deviations or hesitations. Although the function of the Retriever is primarily construction-related, the Sentinel is able to defend itself, or its installation, if necessary. However, Retrievers maneuver poorly and are unable to effectively combat modern Sangheili and human vessels alone. Thus, Aggressor and Protector Sentinels are primarily used as defensive constructs on installations.

Prehistory
Retriever Sentinels were utilized by the Forerunners to aid in the construction of the Halo Array at Installation 00, with the intentions of firing the rings to wipe out the Flood. Retrievers were used as escorts during the Forerunner-Flood war. Following the firing of the Halo Array, Retrievers assisted the keyships with the reintroduction of species throughout the Milky Way, as per the Conservation Measure, accompanying the vessels back to species' homeworlds. Retrievers assisted with the excavation and construction of a massive slipspace portal on Erde-Tyrene, in the present-day country of Kenya, that had been set into motion by the Librarian before her death. These Retrievers were spotted by the human tribe elder N'chala, in the years after the reintroduction, who mistook the sentinels for gods.

Conflict on the Ark
"000 Tragic Solitude"

After the destruction of Installation 04 on September 23, 2552, Retrievers on Installation 00 began to mine the planetoid captured in the Ark's Foundry to construct a replacement installation. On December 11, 2552, these Retrievers were encountered and observed by Sangheili and United Nations Space Command forces during the Battle of Installation 00. The battle ultimately resulted in the Ark becoming heavily damaged, leading to 000 Tragic Solitude—Installation 00's now-rampant monitor—allocating all of his time and resources to the construction of thousands of heavily armed Retrievers that would mine human colonies for resources to repair the Ark, blaming humanity for damaging his installation.

In March of 2555, Tragic Solitude used a captured human to initiate the activation of the Halo Array. As expected by the ancilla, the United Nations Space Command and Swords of Sanghelios reopened the Portal at Voi on Earth to reach the Ark. With the portal now open, Tragic Solitude sent a Retriever through the portal to gather Earth's resources to repair Installation 00, which sustained heavy damages in the aforementioned battle. As the Retriever began to strip mine the country of Kenya, the cruiser immediately responded by attacking the Sentinel. With the aid of several UNSC attack crafts, Endeavor was able to destroy the Retriever. As the UNSC and Sangheili team prepared to venture into the slipspace portal aboard Mayhem, another Retriever suddenly exited—this time, heavily armed. However, the Sentinel was easily outmatched by Mayhem and was destroyed. When Mayhem entered the intergalactic space around the Ark, dozens of Retrievers attacked the corvette and managed to down the ship.

After the crew of Mayhem succeeded in averting the activation sequence of the Halo rings, Tragic Solitude sent his army of thousands of heavily armed Retrievers through the portal to Earth. The Retrievers exited the portal over Kenya and some proceeded to extract materials from Earth, while others immediately engaged the dozens of vessels from the UNSC Home Fleet that were awaiting outside the portal. While the Retrievers' lack of armor made them vulnerable to human fire, their vast numbers overwhelmed the Home Fleet. However, members of the crew of Mayhem fired into the data banks of Tragic Solitude and the monitor reluctantly called off the Retrievers to convince his attackers to stop. The Retrievers retreated back into the portal and returned to the Ark, upon receiving Tragic Solitude's signal. After Solitude's subsequent destruction, the thousands of Retrievers turned dormant, remaining adrift in the space around the installation without any monitor to guide them.

Human appropriation
Doctor Luther Mann theorized that the dormant Retrievers could be reactivated and utilized to repair the Ark, opening slipspace portals to strip-mine uninhabited planets for the resources and minerals required to repair the Ark.

Behind the scenes
ReStrato-Sentinels were originally planned to appear in Halo 3, but were ultimately cut from the game. One of the early settings for the maps "Construct" and "Blackout" was the interior of a damaged Strato-Sentinels in orbit around the Foundry, where other Strato-Sentinels could be seen in the distance harvesting materials from the Foundry's resource moon. However, this idea was scrapped. During production, they were known as "Strato-Sentinels", a name that would later be adopted in subsequent canon. The Strato-Sentinels were also intended to be included in Halo 4, though they were removed from the game during the pre-production stages.

List of appearances

 * Iris
 * Halo 3: The Cradle of Life
 * Halo Legends
 * Origins
 * Halo Mythos
 * Halo: Renegades
 * Halo: Outpost Discovery