The Deathwatch, known also as the "Long Vigil," and the "Long Watch," is a unique Chapter of the Adeptus Astartes comprised solely of Veteran Space Marines that serves the Ordo Xenos of the Imperial Inquisition as its Chamber Militant. They are the warriors of last resort when the Inquisition needs access to firepower greater than that which the Astra Militarum or a team of its own Acolytes or Throne Agents can provide.

Across the galaxy there are innumerable hostile alien civilisations that threaten Humanity, from the green-skinned Orks, to the monstrous Tyranids, sadistic Drukhari, spectral C'tan, and undying Necrons.

It is the sacred task of the Deathwatch to stand sentry against all of these terrible xenos races and many more besides. They are ready to act when such ancient evils rise to threaten Mankind once more. The Space Marines of the Deathwatch form the first, and often only, line of defence against these inhuman horrors.

Unlike other Space Marines, the ones serving in 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 Long Vigil of the Deathwatch is a great honour for any Space Marine. Only the most elite and experienced members of a Chapter are ever chosen for this extremely hazardous tour of duty, the specifics of which must be kept secret by Inquisitorial order and sacred oath even from a Deathwatch Astartes' home Chapter.

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, but if the threat is still too much for even the Deathwatch to handle, the Inquisition will be forced to turn to a full Space Marine Chapter or to multiple regiments of the Imperial Guard to deal with it.