From Halopedia, the Halo wiki
The Promethean Watcher is a Forerunner Sentinel variant. Watchers are part of the mechanical Promethean forces created by the Ur-Didact near the end of the Forerunner-Flood war as a desperate attempt to combat the Flood. Watchers acted as the primary defensive drones of the Promethean shield world Requiem, where they were first encountered by the UNSC in summer 2557.
In the battlefield, Watchers act in a supporting role, assisting Promethean ground units with aerial combat screening and often forcing opponents to engage the Sentinel first. Each Promethean Knight unit carries a single Watcher in the back of their armored carapace, from which the Watcher can be deployed at will. Watchers are essentially low-level ancillas based on the Promethean Knight from which they originate. The watcher's AI is housed in its upper carapace above the "head".
The Watcher is capable of healing and augmenting damaged allies, providing them with hard light shields, and can catch and return enemy ordnance, such as grenades, with a gravity-displacement field. They are armed with a built-in Boltshot. Watchers can resurrect destroyed Knights from the small orb of light they leave behind upon death; however, this can only be performed once for each individual Knight. They are also capable of summoning Promethean Crawlers to the battlefield by conjuring the Crawlers' forms from raw material in the ground. Watchers are also capable of building focus turrets.
Unlike the angular appearance of most Sentinels, Watchers have curving carapaces, a defining characteristic of Promethean war machines. They have a number of appendages, including what appear to be legs hanging below the midsection which serve as stabilizers and two large disks on both sides which provide agile lift systems and a coiled energy expenditure from within them. Most of these segments lack physical joints and are held together by energy fields. Above the midsection there appears to be a head with stylized facial features and also two fins which act as motion curators to control movement. Like the other Promethean constructs, Watchers under the Ur-Didact's control possess orange lighting, in contrast to the blue-lit appearance typical of other Forerunner constructs.
List of appearances