Wars of Vindication
Taking place shortly after the death of Goge Vandire, before his death Vandire had managed to gather a strong following in the Officio Assassinorum through bribery and blackmail. Foremost amongst Vandire's agents was Tziz Jarek, a Callidus assassin. Using the shape shifting powers of Polymorphine, Jarek assassinated the true Grand Master of Assassins and took his place. However, the ever-cautious Grand Master was prepared for such a move and in fact had switched himself with an identical double. Still very much alive, the Grand Master mustered the assassins still loyal to him and assaulted the Assassin Temple on Terra. The vicious fighting eventually spilled into the Imperial Palace itself. The ancient arsenal of forbidden and xenos weaponry was opened up. Gene-sympathetic nerve gases polluted the corridors of the Imperial Palace while neutronic warheads destroyed whole wings of the ancient structure.
In the end, the true Grand Master assassinated Jarek and then disappeared in self-imposed exile. Whether he is still alive as of M41 is not known. In the aftermath of the carnage, the Inquisition set up the Ordo Sicarius at the behest of Inquisitor Jaeger. The Ordo Sicarius has since monitored and overseen the Officio so that such another terrible war can never take place again.