From Halopedia, the Halo wiki
The Promethean Watcher is a variant of the Forerunner Sentinel serving in the Promethean army of the Ur-Didact. They act as primary defensive drones for the Prometheans and their allies in the Halo series.
Watchers act in a supporting role, assisting Promethean ground units with aerial combat screening and often forcing enemies to engage the Watcher first. Deployable from Promethean Knights, Watchers can heal and augment damaged allies, using Hardlight Shields, and catch and throw back enemy ordnance (e.g. grenades). Their sole weapon is the Boltshot.
Watchers appear from the campaign mission Forerunner onwards, where they are seen summoning new Crawlers and protecting Knights and other Crawlers. Watchers are a high-priority target, as if they are not dealt with on time, they can heal other nearby Prometheans. Players should think twice about using grenades near Watchers, as they can throw the grenades back. This is especially fatal if the player uses a Plasma Grenade. However, they cannot deflect heavier explosive projectiles, like those from the Rocket Launcher, Concussion Rifle, Fuel Rod Cannon, or Incineration Cannon. Watchers have small energy shields, and if they are damaged, they tend to fly away.
Killing Watchers with a fully automatic weapon, especially one that has hitscan properties (e.g. Assault Rifle), is highly recommended, as some precision weapons tend to not fire quickly or accurately enough to kill the Watcher before it can regenerate its shields. The red reticle range of the Plasma Pistol is sometimes too short to even make the Noob Combo viable on Watchers, which leaves sniper-type weapons (e.g. Binary Rifle) as other good choices for killing Watchers. If a Watcher is killed quickly enough as it summons Crawlers or Focus Turrets, then the Crawlers or Focus Turrets it tried to summon will not appear. Also, if a Watcher is killed while throwing back projectiles at the player, the projectile it caught will then detonate either on the ground on in mid-air.
It is possible to assassinate a Watcher. This is most easily done when the Watcher is either reviving a Knight, summoning Crawlers, or building a Focus Turret. Look at it so you see both sides of its curved carapace but not its front "face", then jump and hold the melee button (default: RB) to assassinate the Watcher.
In the campaign, Watchers are most commonly seen with other Prometheans. In the mission Infinity and certain chapters of Spartan Ops, though, Watchers can also be seen aiding Covenant troops. Players should prioritize killing Watchers over other enemies quickly because although Watchers may seem weak, they shouldn't be underestimated especially due to their unique defensive and supportive abilities.
Halo 5: Guardians
Changes from Halo 4 to Halo 5: Guardians
Watchers appear from the campaign mission Unconfirmed onwards. Unlike in Halo 4, Watchers are now actually less dangerous, as they are deployed from Knights far less frequently, can no longer summon Crawlers nor build Focus Turrets, are completely unable to revive dead Knights, and are also completely deprived of their energy shields. In addition, Watchers can no longer be assassinated. With many useful abilities being removed, the player can now more easily focus on destroying any enemies in the area before focusing on Watchers, although Watchers should still be dealt with before taking down large bosses like the Warden Eternal, especially on Attack on Sanctum.
Warzone and Warzone Firefight introduce the Legendary and Mythic Watchers, both of which are heavily superior over standard Watchers but otherwise have the same armament. Legendary Watchers are colored orange with turquoise highlights, while Mythic Watchers are colored black and maroon with red highlights.