The last lord of Alamut surrendered to the Mongols in 1256, the Persian historian Juvayni burned the castle's library, and the imam was killed on his way home from the Khan

Rukn al-Din Khurshah succeeded his murdered father as Nizari imam in 1255. In 1256 Hulegu's army reached his castles. Khurshah began taking down towers to show submission. On 8 November 1256 the Mongols surrounded his fortress of Maymun-Diz and bombarded it. On 19 November he came down and surrendered. He ordered all the Nizari castles to give up, and about 40 did. Alamut's commander held out, believing the order was given under duress, then surrendered in December. The Mongols burned and broke the plastered walls and lead-covered ramparts piece by piece. The Persian historian Ata-Malik Juvayni, serving Hulegu, took charge of the library. He says he kept Qur'ans, chosen books and astronomical instruments and burned the rest. He says he read Hasan-i Sabbah's biography and then burned it. He also wrote that no Ismaili should be spared, not even a child. Khurshah asked to go to the Great Khan Möngke in Karakorum. Möngke rebuked him for the castles still holding out and sent him back, and he was killed on the road. His son Shams al-Din Muhammad succeeded him, and the Nizari line of imams continued.