"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

The Deathwatch Space Marines 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 even Throne Agents can provide. Across the galaxy there are innumerable hostile alien civilisations that threaten Mankind, from the green-skinned Orks, to the monstrous Tyranids, sadistic Dark Eldar, spectral C'tan, and undying Necrons. It is the sacred task of the Deathwatch to stand sentry against all of these terrible xenos races. 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.

"We do not hate the alien because he is different, we hate the alien because he had naught but hate in his heart for us."

CATECHISM OF THE XENOS
To be Unclean - That is the Mark of the Xenos
To be Impure - That is the Mark of the Xenos
To be Abhorred - That is the Mark of the Xenos
To be Reviled - That is the Mark of the Xenos
To be Hunted - That is the Mark of the Xenos
To be Purged - That is the fate of the Xenos
To be Cleansed - For that is the fate of all Xenos!

“He who allows the alien to live shares its crime of existence.”