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. Retriever Sentinels. In times of crisis, some of them would assist in combat duties.
The Retriever Sentinel are 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.
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.
List of appearances