"Amongst a hundred men, there may be none fit for the Adeptus Astartes. Amongst a hundred Space Marines, there may be one fit for the Deathwatch."
- — Watch Captain Brand
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 for any Space Marine, as 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. In general, if a group of Deathwatch Kill-teams cannot deal with a xenos incursion, that means that the Imperium has a major conflict on its hands and must deploy a full range of its military forces to meet the threat.
The origins of the Deathwatch are uncertain in current Imperial records, but can be traced to the arrival of the Imperial Frigate Eisenstein at Terra at the start of the Horus Heresy in the early 31st Millennium when a contingent of Loyalist Space Marines from the Traitor Legions arrived in the Sol System to inform the Emperor of Mankind that the Warmaster Horus had betrayed him at Istvaan III and been corrupted by Chaos. It is implied that Battle-Captain Nathaniel Garro and his Loyalist Death Guard Space Marines, along with Captain Iacton "Half-Heard" Qruze of the Luna Wolves, were amongst the first members of a new Imperial organisation created at the order of the Emperor of Mankind that was the ancestor of the current Imperial Inquisition. While not definitive, it seems likely that the contingent of Loyalist Death Guard Space Marines who had arrived at Terra aboard the Eisenstein formed the initial core of what would later become the Deathwatch, hence providing the name for the new Astartes formation.
As the Chamber Militant of the Ordo Xenos, the Deathwatch is tasked with the study and, if necessary, the extermination of dangerous intelligent alien races encountered by the Imperium. They are also tasked with the observation of alien races, and the acquisition of their technology for further study by the Adeptus Mechanicus. This is because the Deathwatch are not merely intended to cleanse xenos cultures from Imperial space. They are also tasked with the recovery and study of alien devices and artefacts. Sometimes it is necessary to use a weapon against the enemy who created it, although this is never done 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; for instance, the C'tan Phase Sword, used by the Callidus Assassins, was recovered from a Necron Tomb World and successfully integrated into the arsenal of the Imperium.
When the call to send Battle- Brothers to the Deathwatch comes, most Chapters select their finest warriors to represent them. The ancient oaths sworn to support the Inquisition and the Deathwatch are taken seriously and any success that a Space Marine displays while serving the Long Watch will be reflected back on their Chapter. These revered champions of the Chapter seek out glory and honour by exemplary service on all missions they participate in. These Astartes are very eager to be part of the Deathwatch and serve the Emperor in this manner.
For some the Chapters, tenure in the Deathwatch can be a time to atone for some transgression committed against the Chapter or its ways. The type of infraction varies from Chapter to Chapter. For those who are strict adherents to the Codex Astartes, simple deviation from the tenets in the sacred text are enough to have a Battle-Brother fall under the unforgiving eye of his superiors. Amongst other Chapters, the infraction is usually much more severe to warrant any sort of sanction. A Space Marine that has been deemed lacking by his superiors has a shadow cast upon him that he must exorcise. Any question of a Battle-Brother’s ability to carry out his duty brings undo scrutiny upon him and those he serves with. These doubts will linger and fester until he is able to redeem himself in the eyes of the Chapter. This redemption can take the form of Crusades, quests, and other heroic endeavours. This can also take the form of an extended secondment to the Deathwatch.
If a Battle-Brother has been sent to the Deathwatch as a means of atonement, then that is usually kept quiet from all but the Chapter’s leadership and the Space Marine in question. Since serving in the Deathwatch is fraught with peril, this is seen as a perfect way to atone in service to the Emperor. The Space Marine will keep his past transgressions closely guarded from his new squad-mates in a Kill-team; if they were aware of these factors then they would surely look at him with suspicion.
The highly perilous and vital nature of their missions means that Deathwatch Kill-Teams have access to exceedingly rare or advanced Imperial equipment, such as Heavy Bolter Gyro Suspensors, M.40 Targeters, and numerous types of specialist ammunition, such as the fragmentation Metal Storm shells or the high-powered Kraken penetrator bolts. If the situation offers no alternative, they will utilize advanced alien weaponry and equipment to accomplish their mission. The Deathwatch often utilises unconventional means of insertion, such as teleportation, high altitude grav-chute drops and Demiurg Termite tanks.
Sometimes, the situation may be more than even a dozen elite Space Marines can handle, and because of this, Deathwatch Space Marines are able to freely requisition any and all Imperial forces they deem necessary to complete their task, from individual Brother Space Marines of other Chapters to entire regiments of the Imperial Guard. A member of the Deathwatch speaks with the full authority of the Inquisition and also possess the unlimited (in theory) authority of that organisation and its servants.
Aside from these details, the armour of a Deathwatch warrior is often decorated with a combination of Purity Seals and holy icons and symbols unique to his parent Chapter. Space Wolves, for example, commonly adorn their armour with all manner of runes and talismans derived from the culture of their homeworld of Fenris, and these can often be seen on the armour of Space Wolves serving in the Deathwatch. Blood Angels Space Marines are known to bear many small teardrop icons, while the Battle-Brothers of the Dark Angels Chapter often carry small winged sword pendants. The longer he serves in the Deathwatch, the more adorned with Purity Seals and devotional scripts an Astartes' armour becomes. Having faced the vilest horrors of the galaxy, the Battle-Brother knows that, ultimately, it is faith that defeats the xenos. While he carries the very finest arms and armour available, spiritual purity is the most deadly weapon the Deathwatch can bring to bear against their foe.
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. Such passages remind the Battle-Brother of his duty at all times, and it is not uncommon for him to chant the lines of such prayers in battle, steeling his heart and those of his comrades against the evil of the xenos.