This a standalone episode on the history of the Ismaili Nizaris and their Order of Assassins.  

The minority sect of Ismailism first arises as the Abbasid Caliphs keep the Shi'a Imams in confinement in Samarra - a hidden religion, secretive and rigidly hierarchical, and professing a unique system of beliefs drawing heavily from Greek philosophy and Gnosticism.  The Ismailis ultimately go on to establish the Fatimid Caliphate, which takes up the mantle of the greatest Muslim state in the world.  But after the coming of the Seljuks and their imposition of Sunni orthodoxy in Iran, the Shi'a are put on the back foot.  And there comes to be founded in the remote mountain fortress of Alamut in northern Iran the Nizari sect of Ismailism - and the famed and fearsome Order of Assassins.