The Ismaili state, also called the Alamut state, was a Shia Nizari Ismaili state founded by Hasan-i Sabbah after he took control of the Alamut Castle in 1090 AD. The state consisted of an extensive nexus of strategic fortresses throughout Persia (Iran) and Syria, surrounded by huge swathes of hostile territory.

The state collapsed when Rukn-ud-Din Khurshah surrendered Alamut Castle to the invading Mongols.

Most Ismaili Shias outside North Africa, mostly in Persia (Iran) and Syria, came to acknowledge Nizar bin Mustansir Billah's claim to the Imamate as maintained by Hasan-i Sabbah, and this point marks the fundamental split between Ismaili Shias. Within two generations, the Fatimid Empire would suffer several more splits and eventually implode.

Following his expulsion from Egypt over his support for Nizar, Hasan-i Sabbah found that his co-religionists, the Ismailis, were scattered throughout Iran, with a strong presence in the northern and eastern regions, particularly in Daylaman, Khurasan and Quhistan. The Ismailis and other occupied peoples of Iran held shared resentment for the ruling Seljuqs, who had divided the country’s farmland into iqtā’ (fiefs) and levied heavy taxes upon the citizens living therein. The Seljuq amirs (independent rulers) usually held full jurisdiction and control over the districts they administered. Meanwhile, Persian artisans, craftsmen and lower classes grew increasingly dissatisfied with the Seljuq policies and heavy taxes. Hasan too, was appalled by the political and economic oppression imposed by the Sunni Seljuq ruling class on Shi'ite Muslims living across Iran. It was in this context that he embarked on a resistance movement against the Seljuqs, beginning with the search for a secure site from which to launch his revolt.

...
Wikipedia