A Chamber Militant is the formal name given by the Adeptus Terra to the military arm of each of the three major branches or "ordos" of the Imperial Inquisition, the secret police and intelligence division of the Imperium of Man.
The Ordo Xenos (Alien Hunters) is served by the Deathwatch Space Marines, the Ordo Malleus (Daemonhunters) by the elite and highly secret Grey Knights Chapter of Astartes and the Ordo Hereticus (Witchhunters) by the Orders Militant of the Adepta Sororitas, better known as the Sisters of Battle.
Usually, these organisations are only called upon by their respective ordo in cases of dire need, when all other options to deal with a given situation have been exhausted.
For example, the Grey Knights are called upon only when a daemonic infestation has grown to the point that the efforts of a lone Inquisitor and their chosen Acolytes and other agents can no longer contain it.
Inquisitors also often make use of specially-trained units of elite Storm Troopers, known as Inquisitorial Storm Troopers.
Some regional Inquisitorial conclaves in various sectors of Imperial space also appear to have their own unique, dedicated military forces to draw upon, such as the the Stormwatchers Space Marine Chapter who are the "sanctioned blades" of the Caligari Conclave.
Sources
- The Inquisition (Background Book), pp. 5, 26-32, 40
- Warhammer 40,000: Inquisitor - Martyr (Video Game), Chapter 1: Campaign Mission 5 - Madness Incarnated