The fortress of Girdkuh in Persia (Iran) surrendered to the Mongols in 1270. Girdkuh was the last stronghold to surrender to the Mongols, who had invaded Persia and had captured Alamut and other strongholds in 1256, causing a collapse of the Nizari Ismaili state. Much of the Ismaili literature of the Alamut period was destroyed with the collapse of the state and very little was written or preserved by the community in the following two centuries. What we know about the Ismailis of this period is largely derived from non-Ismailis historians of the time, who were for the most part hostile to the Ismailis.
In order to avoid persecution, the Ismailis sought refuge under the cloak of Sufism. Nizari Quhistani was perhaps the first author of this period to have adopted Sufi idioms to convey the Ismaili doctrines. His writings reflect the earliest instance of a literary blending between Nizari Ismailism and Sufism in Persia. There is only one complete surviving copy of his collected works, the Kulliyat, which is now preserved in St. Petersburg, Russia.
— Source: Farhad Daftary, The Ismailis Their history and doctrines (Cambridge University Press, 1990)