Ordo Sicarius
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), 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) requested that an Order of the Inquisition was formed to monitor the Assassins. Thus was formed the Ordo Sicarius, to root out any corruption within the Office of Assassins.
As the Assassinorum works secretively, each of the Assassin Temples contains a number of Assassin-Inquisitors to monitor them from within. One has to wonder how an Inquisitor Eversor would work...
The Ordo Sicarius has successfully uncovered and prevented two assassination attempts on the High Lords of Terra; considering that each assassination requires a two-thirds vote from the High Lords, the fact that they needed prevention is... odd. (The intention is that the assassin's were traitors and not acting on the will of the High Lords. It does lead to interesting possible political intrigue though.)
Nothing to do with the other Sicarius.
It should also be noted that it has been spelled Ordo Sicarus (without the second "i"), although it seems the "Sicarius" is the official spelling now.
