By the eleventh century, the great Abbasid empire was beginning to crumble.  The Caliphs had been weakened, the Turkish military dominant.  On the periphery of the empire, rebellious groups were forming new states.  Among these, the Isma'ili Shi'a would become the most powerful, establishing a rival caliphate that overshadowed the Abbasids.  The line of Isma'ili Imams continues to this day.  In this episode, we look at the Isma'ili Shi'a: where they came from, what they believe and why they posed a threat to the Abbasid Caliphate.