This is a subject that has come up as part of a side project that will, hopefully, see the light of day soon.
Earlier this year we discussed the twilight of the chambers militant. Looking beyond the exact nature of the relationship between the various Ordos and their "chambers militant," this discussion focuses more on who those chambers might be for a number of the Ordos. I don't have a big stake in a number of the Ordos - those whose interests are least likely to result in them being engaged in major battles.
The lore regarding the various Ordos and the chambers militant has evolved and shifted over the years. I'll use the Ordo Hereticus and the Adepta Sororitas as my example:
THE ORDO HERETICUS
At the end of the Age of Apostasy, the Inquisition formed a secret order within its ranks to watch the Ecclesiarchy more closely. While the Inquisition as a whole monitors every aspect of the Imperium, the Ordo Hereticus is primarily concerned that another Plague of Unbelief does not occur. Although the Ecclesiarchy regulates itself and its followers closely (the Adepta Sororitas are most active in this task), the Ordo Hereticus provides another line of defence to ensure that those in power do not abuse their position to such an extent that it threatens the stability of the Imperium again.
These Inquisitors monitor Wars of Faith closely to ensure the objectives laid down by the Ecclesiarch are both justified and not exceeded by those actually fighting. They ovesee the Frateris Militia to safeguard the spirit of the Decree Passive (if not its letter) and to ensure its prohibitions stay enforced. They also regulate the amount of wealth and territory claimed by individuals within the Ecclesiarchy and halt attempts by Cardinals to amass more power than the Ordo Hereticus deems appropriate.
...Rising from the ashes of the Age of Apostasy, when the insane Lord Vandire plunged the Imperium into one of the bloodiest periods in its already bloodstained history, the Ordo Hereticus are the guardians of Humanity's future. In conjunction with their Chamber Militant, the Adepta Sororitas - also known as the Sisters of Battle - the Ordo Hereticus hunt down and destroy the most insidious threats to Humanity, the enemy within: witchcraft, heresy and mutation.
A shadowy branch of an organisation already swathed in secrecy, the Ordo Hereticus is the guardian of Humanity, the watcher at the gates that protects Mankind - as much from its own weaknesses as any threat from without. The Witch Hunters of the Ordo Hereticus are sinister, feared individuals. Their skin is pale from long days spent in the excruciation chambers of Inquisition fortresses or poring over ancient texts that speak of the heretic and his machinations.
For the first few millennia of its existence, the Ordo Hereticus was but a rumour even to other Inquisitors, so fanatical was its secrecy guarded. All this came to an end following the Age of Apostasy, when it was decided that the Ordo would serve better by casting an intimidating shadow, rather than existing as a myth too easily ignored. As the Ecclesiarchy flourished, so too did the Ordo Hereticus grow with it, for nothing piques an Inquisitor's suspicion more readily than a holy man claiming to speak with the Emperor's voice. Often, its againts pose as servants of the Ecclesiarchy, feigning subservience to a Cardinal's will until they have sufficient proof of the man's guilt - it is a rare priest with whom a member of the Ordo Hereticus cannot find fault.
...In addition to monitoring the Ecclesiarchy - ensuring that Wars of Faith do not exceed their mandate or that its many Cardinals do not amass more power than is deemed appropriate - the Ordo Hereticus keeps a close eye on many other Imperial organisations...
The 6th Edition Codex: Inquisition also shows that the Ordo Sanctorum, founded in M36, is responsible for monitoring the Ecclesiarchy.
This leads to an interesting overlap/duplication. The Ordo Hereticus was, per the original 2nd edition lore, founded specifically to watch over the Ecclesiarchy. That shifted in 3rd edition, when the Ordo Hereticus watched over witches, heretics, and mutants (not the Ecclesiarchy). It shifted back again in 6th edition, combining both versions of the Ordo Hereticus (watching over the Ecclesiarchy and witches, heretics, and mutants). And that edition also gave us the Ordo Sanctorum, which watches over the Ecclesiarchy.
So it appears that we either have two separate Ordos that watch over the Ecclesiarchy, or the Ordo Sanctorum is an Ordo within the Ordo Hereticus. With the Byzantine nature of the Imperium, and especially within the Inquisition, both cases might apply concurrently (don't think too hard about it or you'll get a headache ).
My concern, though, resides with the chambers militant of those orders. The original lore for the Inquisition and the chambers militant of the various Ordos was something along the lines of each ordo having a Space Marine Chapter at its disposal as its chamber militant. The easy examples of this are the Grey Knights, we serve as the chamber militant of the Ordo Malleus, and the Deathwatch, who serve as the chamber militant of the Ordo Xenos. Codex: Witch Hunters shook things up a bit when it told us that the Adepta Sororitas were the chamber militant of the Ordo Hereticus.
Oddly, more recent works have downplayed the relationships between the chambers militant and their respective Ordos. Where the 3rd edition lore implied that the chambers militant were subordinate to (and potentially part of) their Ordo, the more recent lore has re-established the relationships on more of a peer basis, where the chambers militant serve with and alongside their Ordos due to the alignments in their interests, without being subordinate to them. The Adepta Sororitas, as part of the Ecclesiarchy, were never truly subordinate to the Ordo Hereticus, so this shift in lore didn't really affect them much. The most telling issue, though, is that the current lore doesn't really mention the "chambers militant" much.
So for the purposes of this discussion, when I refer to "chambers militant" I mean those military forces that serve alongside the various Ordos. Based on the current lore, I'm assuming they're separate from their Ordo, but strongly aligned with them.
With the original lore about the chambers militant being Space Marine Chapters rendered inaccurate (due to the Adepta Sororitas), the question then becomes: Of the known military forces of the Imperium, what would be the most appropriate chamber militant for each of the Ordos of the Inquisition?
Two somewhat ubiquitous choices are the Inquisitorial Storm Troopers (or perhaps some Inquisition variant of the Militarum Tempestus Scions) and the Adeptus Astartes (i.e., not a dedicated Chapter, per se, but any Chapter or a number of Chapters whose interests align with the Ordo). The Red Hunters are a ready example of a Chapter of the Adeptus Astartes with strong ties to the Inquisition as a whole (apparently). For my purposes, if we can't identify a specific force that would be suitable as the chamber militant of an Ordo, I'm assuming that it's either the Adeptus Astartes (either a specific Chapter or simple reliance on the various Chapters when force is needed) or the "Inquisitorial Storm Troopers."
And we also have to keep in mind that an Inquisitor is empowered to commandeer the aid of any individual or group within the Imperium.
My initial interest is about the Ordo Hereticus/Ordo Sanctorum. If we assume that the Ordo Sanctorum has specific responsibility for the Ecclesiarchy, then would the Adepta Sororitas shift over to the Ordo Sanctorum as their dedicated chamber militant? It wouldn't be inappropriate for the Adepta Sororitas to fight alongside the Ordo Hereticus when the situation allows/demands. A counterpoint (regardless of who has primacy in responsibility over the Ecclesiarchy) is that the use of an element of the Ecclesiarchy to enact military action against the Ecclesiarchy might seem a bit ill-conceived. In this, I could see the Ordo Sanctorum having some military force other than one aligned with the Ecclesiarchy as their chamber militant.
I'm going to look at a few other Ordos for which we have little information...
The Ordo Custodum, founded in M35, is stationed on Holy Terra for some undisclosed purpose. The name and location present the possibility of some Ordo that watches over the Adeptus Custodes (good luck with that job). If that is indeed the case, the only military force I can think of to deal with the Adeptus Custodes is the Adeptus Custodes themselves. Perhaps this is an aspect of the duties performed by the Eyes of the Emperor. Regardless, this isn't an Ordo that I think has much play in the game (assuming they are confined to Terra).
The Ordo Machinum watches over the Adeptus Mechanicus. I could see them using Skitarii, Inquisitorial Storm Troopers/Militarum Tempestus, or Adeptus Astartes (especially since they often work closely with the Ordo Xenos).
The Ordo Militarum, founded in M35, monitors the Astra Militarum. It would be easy to envision this Ordo having Inquisitorial Storm Troopers/Militarum Tempestus, with an Ordo Militarum Inquisitor posing as a Commissar or other officer.
The Ordo Sicarius watches over the Officio Assassinorum, so it might be logical to assume that the Inquisitors of this Ordo make use of Assassins as their chamber militant. We can look at history, though, when large forces of the Adeptus Astartes had to deal with Assassins (those turned out be very bloody and costly affairs for everyone involved).
Oddly, there is no Ordo listed that watches over the Imperial Navy. I can't imagine that there isn't such an Ordo, however. If such exists, I imagine that, as with the Ordo Militarum, that Ordo would make use of the most highly trained examples of the military forces of the Aeronautica Imperialis, with both pilots and infantry (veteran examples of the Voidsmen from Kill Team: Rogue Trader, perhaps) acting as their chamber militant.
Also, there isn't an Ordo identified as scrutinizing the Adeptus Astra Telepathica. If there is such an order, I imagine that the Sisters of Silence would make a suitable chamber militant.
Do you agree or disagree with any of these assessments? Are there other Ordos that you think bear examination?