Deathwatch army rules, detachment abilities, stratagems and lore for Warhammer 40k, with links to the full unit datasheet table.
The Deathwatch, known also as the "Long Vigil," and the "Long Watch," is a unique Chapter of the Adeptus Astartes comprised of mostly 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.
The exact origins of the Deathwatch are uncertain, but its inception can be traced to the period known as the Time of Rebirth in 544.M32, and the arrival in Imperial space of the largest Ork WAAAGH! seen since the Ullanor Crusade during the closing years of the Great Crusade.
Led by the mysterious warlord known only as The Beast, this Greenskin invasion threatened the very Throneworld of Terra itself. During this conflict, remembered as the War of the Beast, it was the newly-elected lord commander of the Imperium, Chapter Master and sole surviving member of the Imperial Fists named Koorland who devised the concept of utilising small, veteran Astartes kill-teams to eliminate vital strategic assets of the encroaching Greenskins and eventually, The Beast itself. Though reluctant at first to acquiesce to the lord commander's plan to approve the creation of such a force, desperate times called for desperate measures. Therefore, the High Lords of Terra consented to Koorland's proposal.
The original recruits for this newly created elite force were drawn from the survivors of the various Chapters who took part in the initial invasion of The Beast's capital world of Ullanor. These Chapters suffered severe losses during the invasion. Thus standing in vigil over their fallen brethren, the seeds of the Deathwatch were sown.
Lord Commander Koorland eventually came to an agreement with the Inquisitorial Representative Wienand to assuage any doubts by the rest of the Senatorum Imperialis that the Deathwatch would fall under the purview of the Inquisition, but an Astartes would ultimately serve as Chapter Master.
It was in the wake of the War of the Beast that the Inquisition itself was divided into the first two of the Ordos Majoris, the Ordo Malleus and the Ordo Xenos. The Deathwatch was chosen to be an integral part of the newborn Ordo Xenos and serve as its Chamber Militant the way the Grey Knights served as the Chamber Militant of the Daemon-hunting Ordo Malleus.
However, there are conflicting accounts which state the Deathwatch's origins occurred somewhat differently, and that it was founded at an unknown time centuries ago. These records state that a conclave of Inquisitor Lords -- the Apocryphon Conclave of Orphite IV -- convened with the sole purpose of formulating an Imperium-wide strategy to combat the many xenos threats that assailed Mankind.
The members of this conclave were, in the main, drawn from the ranks of the Ordo Xenos, all of whom shared the belief that one day, there would come a time when Humanity would be consumed by slavering xenos beasts, or enslaved to creatures of unutterably alien origins. They foresaw an age when every alien life form in the galaxy, and others from beyond it, might rise up and the Age of the Imperium would come to an end. They had no inkling when such a terrible age might come, but they knew with dreadful certainty that it surely would, and they determined that no effort should be spared to avert such an end.
The conclave sat for many standard years, and bitter debate raged back and forth between Inquisitor Lords of many different positions. Some believed that every trace of alien life should be purged from the galaxy, while a few advocated the formation of alliances with those alien races like the Craftworld Aeldari that could be tolerated. Some spoke of the existence of ancient, god-like beings that have slumbered since before the birth of Mankind awakening to enslave the galaxy, while others foresaw that death would come at the hands of invasion from other galaxies entirely.
Lore excerpt from Warhammer 40k Wiki on Fandom, available under CC BY-SA 3.0.