The Deathwatch are a unique and specially trained Space Marine Chapter that dedicates its every hour to xenos hunting. They are the Chamber Militant of the Ordo Xenos, charged with protecting it in its search of information, containment, and ultimate destruction of all xeno species.
Rather than following the standard Codex Astartes organization, the Space Marines themselves are drawn from many different chapters. These chapters have sworn sacred oaths to raise a unit of Space Marines specifically for this task. The warriors are then gathered together to fight where and when they are needed.
Giant fortresses orbit desolate worlds at the edge of the galaxy, keeping constant vigil over possible xenos forces. There are also several secret bases spread throughout the Imperium, which provide a launch site for their crusades.
The warriors who fight are given the honour of painting their armour black (to show their service to the Emperor), while leaving one shoulder pad with the original insignia of the Chapter from which the marine came. Although the armour is painted black, it is never all painted black as this action would dishonour the armour’s Machine Spirit. They also are equipped with a new shoulder pad with the symbol of Deathwatch onto their other side. Once in the employ of the Deathwatch, there is no set length of time for service, rather it is as long as the commander deems necessary. Each Space Marine may serve a discrete amount of time, or for the duration of a mission, which may be a number of years. Once the mission is complete, the Space Marine is allowed to return to his chapter, under an oath of silence, as their duty has been fulfilled.
The orders of the Deathwatch are not merely the cleansing of xenos cultures. They also include the recovery and study of alien devices and artifacts. Sometimes it is necessary to use a weapon against the enemy who created it, although this is not taken lightly. The Deathwatch are constantly vigilant for sabotage, or to advise if it is truly safe to use a weapon of xenos origin. The Adeptus Mechanicus are always on the lookout for alien technology; the C’tan Phase Sword, used by the Callidus assassin was recovered from a Necron tomb world and successfully integrated into the arsenal of the Imperium.
Usually, a Deathwatch team is led by an Inquisitor, but in extreme circumstances, a Deathwatch Captain or Librarian may take command of the unit. Their word is law, and can requisition anything they desire to get the job done. They are the best armed and trained units in the Imperium. They might be deployed in areas where conventional methods are insufficient or if the troops available are not properly equipped or trained for the task. The presence of a Deathwatch team is always welcomed, even though they are feared by all those who know of them.
The Deathwatch is a secretive group of Xeno hunters to put it bluntly. Keep in mind that if you are playing a Space Marine that is part of the Deathwatch, you took an oath of silence. Because of this there are certain things that should never be reveled to anyone (save the Emperor and the Primarch when you go to the gates of the after-life) who is not part the Deathwatch, wither it be your Chapter-kin, or even an Inquisitor of another Ordos. The Deathwatch specialize in xenos termination. Despite this, you may be sent on a mission to aid a xenos that the Inquisitor has an alliance with, to hold up the Inquisitor’s side of the agreement or to re-forge the alliance. In this rare case, remember that the Inquisitor’s word is law, no matter how hard it may be to follow.