The Ordo Sicarius, who - following the nicknaming conventions of the other Orders - would presumably be the Assassinhunters (or the better sounding, although less logical Manhunters, or if you want the manhunterhunters), are an Order of the Inquisition formed after the Age of Apostasy and the Wars of Vindication which threw the Officio Assassinorum into chaos (small "C"). During the reformation of the Officio, Inquisitor Jaeger ("hunter", subtle G.W) proposed a number of safeguards be placed upon the Assassins so that an eventuality such as the Wars of Vindication or The Beheading that preceded it would never happen again.
- The Assassin Temples would be decentralised throughout the Imperium so that they could not become corrupted or fall to heresy if any of the others fell.
- All Assassins (Eversor excepted) would be subject to regular psycho-indoctrination to maintain their loyalty.
- All missions performed by assassins will be followed by a detailed report, subject to the scrutiny of the Inquisition.
- Any deployment of an assassin will require the sanction of a two-thirds majority vote by the High Lords of Terra.
- Finally, an Ordo of the Inquisition was formed to monitor the assassins to root out any corruption within the Office of Assassins; thus was formed the Ordo Sicarius.
As the Assassinorum works secretively, it's adherence to the above requirements would be limited to how much the Imperium trusted the Assassinorum to actually stick to them, since they could otherwise be flaunting the rules, assassinating everyone and sundry, making it look like an accident and no-one would be the wiser. Therefore the only way to effectively monitor each of the Assassin Temples was to embed a number of Assassin-Inquisitors to monitor them from within; these agents may not necessarily become (or even qualify as) "Temple Assassins" themselves, but considering that the Officio Assassinorum has considerably more resources and staff than just the temple assassins, it would be much easier to infiltrate the organisation under the guise of a analyst, instructor, artificer, surgeon or any other role. One would hardly expect an Inquisitor to become an Eversor for example. The better question becomes whether the Officio is actually aware of which agents belong to the Inquisition, and what information they allow the Inquisition access to.
The Ordo Sicarius has successfully uncovered and prevented two assassination attempts on the High Lords of Terra; considering that each assassination officially requires a two-thirds vote from the High Lords, the fact that they needed prevention is... odd. However as more and more fluff on the High Lords has been released, it has been shown that some, but probably most if not all of the agencies of the Imperium (not just the Officio Assassinorum) have operatives spying on and manipulating each other all the time, and are not adverse to bloodying each other over conflicts of interest and no-one bats an eye. A potential plot against a High Lord directly would be scandalous, but probably not out of the consideration of the agencies if they thought they could get away with it. Even the Chancellor of the Imperial Senate has agents within the other organisations, and he's not even a High Lord, though he had enough power that such an astute politician as Roboute Guilliman found him useful. Not all of the agents of the Officio Assassinorum are actually "Temple Assassins" and in fact they have access to considerably more resources than that. The assertion seems to be that the Officio Assassinorum is simply the best at infiltrating other Adeptus, and is the best as rooting out foreign from within, the fact that the Ordo Sicarius even uncovered the plot would have been a tremendous victory for them.
The Two-Thirds Rule
A lot gets made by fa/tg/uys about the requirement for two-thirds assent from the High Lords before an assassin is deployed, even when this becomes impractical or so far below the purview of the High Lords themselves that one would wonder why it would matter (such as killing Xenos, or enemies of the state) or gets used as evidence that the High Lords had something to do with it whenever an Assassin kills somebody, but this is not always the case.
Assassins have considerable leeway when they are undertaking their assignments: Just because an Assassin wastes a priceless piece of munitions on you does not mean that the High Lords wanted you dead, or even know your name. This is not merely true for the nameless bodyguards protecting the target, but there are documented cases of Assassins changing their mission parameters based on shifting circumstances (like the original target being dead by the time they arrive) and then chewing their way through planetary leadership to achieve their objective. This is probably why the Inquisition gets detailed reports following a mission, to ensure that the Assassin hadn't reached beyond their original remit in the execution of their target(s) and to mitigate against unexpected collateral damage.
These rules also seem to get bent when it is necessary to do so, as found during the War for the Golden Cog when the Fabricator General claimed ownership over the Fortress of the Soulless on Terra. While the Culexus assassins had every right to defend themselves, it was a snipers bullet that managed to avert a larger civil war between Mars and Terra. If the deployment of a Vindicare assassin to the situation was officially authorised, you can bet that the Fabricator General was not present during that vote.
The older Codex: Inquisition as well as the Inquisitor (RPG) both provided the rather commonsense detail that these edicts have been forged by the Ordo Sicarius itself in matters of political expediency, reasoning that since the Imperium is so huge, such abuses of power become necessary, and in any case if the Assassin does their job well then no-one would have to know about it. This is backed up in newer fluff in the case of the attempted assassination of Gabriel Seth by an Eversor, Seth was presented with an Inquisitorial sigil as justification for the attempt with the claim that he had made powerful enemies. As well as the actual assassination of Chapter Master Dubaku of the ill-fated Celestial Lions. Hell, even Lord Solar Macharius was slain by the Officio Assassinorum on the orders of the Inquisition; the assassin then later killed the Inquisitor to cover up the truth.
- It has been spelled Ordo Sicarus (without the second "i"), although it seems the "Sicarius" is the official spelling now.
- Nothing to do with the planet Sicarus.
- Also has nothing to do with that Sicarius. Although an entire Ordo dedicated solely to rooting out the corruption within Cato Sicarius would probably be endlessly amusing.