The Brotherhood of Luna is a small sub-order of Grey Knight stationed on Luna. They guard a shrine on its surface, where it is said that an ancient daemonic presence was banished during the opening battles of the Horus Heresy. The specifics of this have been lost save perhaps to the most secure archives on Titan, but the Brotherhood of Luna take their duty seriously, viewing any daemonic presence so close to Terra as an incalculable risk. As such, they specialize in preventing daemonic incursions, rather than countering them once they have occurred.
Over the long millenia of their existence, the Brotherhood of Luna has discovered that certain rituals can strengthen the barrier between realspace and the Warp before a rupture occurs. This invariably requires the ritualized torture of sentient beings. Most often this takes the form of flaying specific runes into their skin, though it may also include exsanguination or immolation. While it is most efficient to conduct these rituals upon captured Chaos cultists, faithful worshipers of the Emperor can also be used if the situation demands it (the Brotherhood of Luna have also had good results with Eldar when they can get them).
These rites are often gruesome and are perceived as unnecessary by other bodies of the Imperial military as they occur before any daemonic presence is evident. In fact, if they are successful, all signs of daemonic influence are prevented. Thus, the Brotherhood of Luna have done much to propagate the Grey Knights' reputation as callous and brutal warriors. Only within the Grey Knights is their value acknowledged, for every Grey Knight understands the worth of sacrificing dozens to prevent the deaths of planets.
The Brotherhood of Luna wear light grey ceramite rather than the silver of the main Grey Knight Brotherhoods. This is in honor of their long vigil on the grey dust wastes of Luna.
Recently, the Prognosticars of the Grey Knights have determined that significant Warp disturbances have followed the Executioner's fleet currently crusading in the Liani system. This may be linked to the mysterious disappearance of a Grey Knight task force on the nearby planet of Roark's Conundrum. Fearing a daemonic outbreak in the system, the Grey Knights have dispatched a small force of the Brotherhood of Luna to shadow the Executioners...