Space Marines of the Deathwatch serve the Ordo Xenos of the Imperial Inquisition as its Chamber Militant – the warriors of last resort when the Inquisition needs access to firepower greater than the Imperial Guard or a team of its own Acolytes or Throne Agents can provide.
In essence, the Deathwatch is tasked with the study and, as often necessary, the extermination of dangerous intelligent alien races encountered by the Imperium.
Unlike other Space Marines, the Deathwatch are not truly a separate Chapter of the Adeptus Astartes; rather, they are a collection of veteran Space Marines drawn from all of the different extant Chapters who serve together in the Inquisition’s service for a discrete period of time. To be chosen by one’s Chapter to serve in the Deathwatch is a great honour, as only the most elite and experienced are ever chosen for this extremely hazardous tour of duty. The specifics of this service must be kept secret from even a Space Marine’s home Chapter, both by Inquisitorial order as well as sacred oath.
Upon being seconded to the ranks of the Deathwatch by his parent Chapter, a Battle-Brother enacts a ritual in which his Power Armour is repainted black. Not all of the armour is repainted, however, for the right shoulder plate is left in its original colour so that the Astartes’ original Chapter may be known.
The shoulder plate and the entire left arm is electroplated silver, and polished to a high shine. At the centre of the plate is the icon of the Deathwatch — the ancient Inquisitorial “I”, mounted with a death’s head and crossed bones. Around the icon, passages from various devotional texts are engraved, including the Catechism of the Xenos and the Third Abjuration of Terra.
Deathwatch Space Marines do not usually form the standard tactical groups like squads and companies generally used by the Adeptus Astartes. Instead, they operate as small special forces units in close-knit groups of specialists called Kill-Teams. If a xenos threat is particularly dangerous, several Kill-Teams may be assigned to deal with it.
In general, if a group of Deathwatch Kill-Teams cannot deal with a xenos incursion, the Imperium has a major conflict on its hands and must deploy a full range of its military forces to meet the threat.
The highly perilous and vital nature of their missions means that Deathwatch Kill-Teams have access to exceedingly rare or advanced Imperial equipment and numerous types of specialist ammunition.