Crawlers are small, quadrupedal machines capable of both ranged and close-quarters combat, using a built-in Z-110 Boltshot as their primary weapon; this weapon disintegrates targets to prevent their biomass from being co-opted by the Flood. Unlike the majority of Forerunner automatons, they lack anti-gravity capabilities of any kind and instead rely on their four legs for locomotion. They are however capable of vertically scaling structures and leaping great heights in order to get tactical advantage. Crawlers are ferociously aggressive, having been specifically designed to hunt down and eliminate any foreign threats, their inorganic nature making them immune to Flood infection. They travel in large packs and work together to destroy enemies by overwhelming them from every conceivable angle, relying on skirmishing tactics.
Crawlers are often directed by Promethean Knights, who regard them as cannon fodder. They are protected by Watcher Sentinels, which can offer them cover in the form of small hardlight shields or create new Crawlers from raw materials in the ground. In addition to the basic type of Crawler, a more heavily armed unit and a long-range unit are also employed.
The Ur-Didact originally designed the Crawlers for his Prometheans to use against a number of infantry threats, and, eventually, the Flood. They were created to target and destroy organic matter, overcoming their enemies with numbers. It has also been theorized that they were sent into Flood strongholds to soften their infantry numbers before more valuable assets were sent in. Numerous Crawlers were encountered by UNSC and Covenant personnel on the shield world Requiem in mid-2557 during the Battle of Requiem, and later during the Requiem campaign in February 2558. Crawlers also accompanied the Didact during his attack on the human forces on Installation 03 and at the Composer's Forge. By October 2558, Crawlers had been taken control by Cortana and the Warden Eternal to serve in the Created, during which Crawlers sported a dark gray body color with bright red highlights.
Crawlers exhibit animal-like behaviors. They can be found "sleeping" outside of combat, scratching themselves with a back leg similar to wolves or dogs, and even showing distress by whining if kept isolated from other Crawlers.
List of appearances