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Strato-Sentinels are part of a group of Forerunner Sentinels that were made to build structures or assist/oversee with these tasks. They also performed tasks relating to this such as gathering materials e.g. Other tasks include terraforming projects, and are capable of mining moons, asteroids, or other planetary bodies. They were notably used to help in the construction of the Halo rings, such as the newly constructed Installation 08 in 2552 in the modern era.
Dark gray in color, Strato-Sentinels 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.
- Main article: Assembler Sentinel
- Main article: Excavator Sentinel
- Main article: Extractor Sentinel
- Main article: Retriever Sentinel
The Retriever Sentinel is considered to be one of the more powerful variants of the Sentinel, Retrievers are massive, ship-sized drones that were utilized to construct installations of the Halo Array from Installation 00, as well as to provide maintenance to the Ark itself.
- Main article: Steward Sentinel
Strato-Sentinels were used as escorts during the Forerunner-Flood war. Strato-Sentinels were also used at Installation 00 at its stripmining moon, harvesting precious raw construction materials. Following the firing of the Halo Array, Strato-Sentinels assisted the keyships with the reintroduction of species throughout the Milky Way, as per the Conservation Measure, accompanying the vessels back to species' homeworlds. Strato-Sentinels 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 Strato-Sentinels were spotted by the human tribe elder N'chala, in the years after the reintroduction, who mistook the sentinels for gods.
Strato-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.
Vic DeLeon indicated that the Strato-Sentinels in Halo 3 were intended to be of a similar size to the similarly-cut Guardian enemy in the cut Guardian Forest level. This level was intended to feature the Strato-Sentinels heavily, with one rising out of the ground with the player riding it. Further information provided by campaign lead Rob Stokes indicates the Strato-Sentinels would have been featured in the ending of Guardian Forest wherein the Master Chief would activate them to destroy Truth's fleet above The Ark, as well as in the ending of Forerunner City and Halo, in both cases being shown constructing Installation 08. 
Concept art of the Strato-Sentinels in Halo 4.
Strato-Sentinels in "Origins"
List of appearances
- Halo 3: The Cradle of Life (First appearance)
- Halo Legends
- Halo: Hunters in the Dark
- Halo Mythos (Mentioned only)
- Halo Wars 2
- Halo: Renegades (Mentioned only)
- Halo: Outpost Discovery
- Halo: The Essential Visual Guide, page 167
- The Art of Halo 3, page 32
- Halo Legends, Origins
- Halo: Hunters in the Dark, Chapter 6
- Halo Encyclopedia (2009 edition), page 259 (2011)
- Halo Encyclopedia (2022 edition), page 328-329
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- Halo 3: The Cradle of Life
- Twitter, franklez - "They are deliberate variants created by the Studio4°c in partnership with us - and they are canonical and in that sense those variants could show elsewhere. They are tweaked as new designs to infer re-seeding and general array and repopulation maintenance duties. See also Iris"
- Halo Encyclopedia (2022 edition), page 343
- Halo Encyclopedia (2022 edition), page 356
- Halo Wars 2, Phoenix Log: Retriever Sentinel
- Canon Fodder: Outpost Discoveries
- Halo:Encyclopedia, p. 166 (2009 edition), p.176 (2011 edition)
- Halo 3: The Cradle of Life, page 2
- Bungie.net: Deconstructing Construct
- The Art of Halo 3, page 128
- Awakening: The Art of Halo 4
- Twitter, Vic DeLeon: "Fwiw this is almost exactly what the Guardians looked like back when they were first imagined on Halo 2, only they were much larger, almost like the Strato-Sentinel." (Retrieved on Jun 1, 2020) [archive]
- Twitter, Dan Miller: "#halo3 original 'guardian forest' (cut map):" (Retrieved on Jun 1, 2020) [archive]
- Work of Rob Stokes, Halo 3 (Retrieved on May 25, 2023) [archive]