From Destinypedia, the Destiny wiki
Watchers are security drones that do not have any direct attack methods, but instead have detection gazes that look out for intruders. Once it locates a target, it will fully shield itself and call for Cabal reinforcements. These Cabal must be killed to disable its shield. Once its shield is down, it can then be killed, but it will detonate upon death, killing anyone that is close enough to the explosion. A door leading to a reward chest will open if they are killed fast enough without raising the alarm or having activate their shields. If the team fails this they can close the entry door which will make any Cabal killed plus any Watchers killed respawn so you can try again.
Note, to get the reward chest you must have the correct key otherwise you won't be able to unlock the chest so encounter with caution.
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