Retriever Sentinel

"It's a Strato-Sentinel. It's one of the automated drones the Forerunners use on their artificial worlds. The Covenant originally thought them to be holy warriors of the Sacred Rings. There are many types of Sentinels that the Forerunners created—this particular one is designated as a Retriever."

- Usze 'Taham

The Retriever, also known as a Strato-Sentinel, is a Forerunner Sentinel variant. Retrievers are designed to perform terraforming projects and capable of mining moons, asteroids, or other planetary bodies, such as the resource moon located in the middle of Installation 00.

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

Dark gray in color, Retrievers are massive Sentinels that are slightly smaller than 498 meters. 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.

When a Retriever is threatened or attacked, the Sentinel variant may use its gravitic beam as a dangerous, though cumbersome, weapon. 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 one of the primary purposes of a Retriever is 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.

The Forerunners utilized Retrievers to help in the construction of the Halo rings, such as the newly constructed Installation 04 in 2552. The Retrievers gathered raw materials from the mined planetoid in Installation 00's planetoid 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 if necessary. However, Retrievers maneuvere poorly and are unable to effectively combat Sangheili and human vessels alone.

History
"000 Tragic Solitude"

They were also used during the construction of the Portal at Voi on Erde-Tyrene, in the present-day country of Kenya. Some Retrievers apparently assisted the Forerunner keyships with the reintroduction of species throughout the Milky Way.

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. Directly after the battle resulted in the Ark becoming heavily damaged 000 Tragic Solitude—Installation 00's now-rampant monitor—allocated 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.

In 2555, 000 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, 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 managed to avert the activation 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.

Trivia
Retriever Sentinels were originally planned to appear in Halo 3, but were cut from the game. During production, they were known as "Strato-Sentinels".

List of appearances

 * Iris
 * Halo 3: The Cradle of Life
 * Halo Legends
 * Origins
 * ''Halo: Hunters in the Dark
 * Halo Wars 2