In the early 9th Century many Muslim factions believed in the idea of a 'Mahdi', typically the last leader of their faction, who had been taken away by God but would at some point return as a type of Messiah. This disappearance was known as 'Occultation'. Several prominent Ismaili figures taught that this had happened to Muhammad ibn Ismail, the son of the Imam whose succession had been debated.
However, at the turn of the 10th Century a Syrian named Al-Mahdi announced that Muhammad ibn Ismail and his descendants had in fact been hiding to protect themselves