Assassins (from Arabic:أساسيون‎ Asasiyun) is the name used to refer to the medieval Nizari Ismailis. Often characterized as a secret order led by a mysterious "Old Man of the Mountain", the Nizari Ismailis were an Islamic sect that formed in the late 11th century from a split within Ismailism, itself a branch of Shia Islam. In time, the Nizaris began to pose a military threat to Sunni Seljuq authority within their territories by capturing and inhabiting many unconnected mountain fortresses throughout Persia (and later also Syria) under the leadership of Hassan-i Sabbah (who is typically regarded as the founder of the Assassins), therefore founding the so-called Nizari Ismaili state. While "Assassins" typically refers to the entire medieval Nizari sect, in fact only a class of acolytes known as the fida'i actually engaged in assassination work. Lacking their own army, the Nizari relied on these trained warriors to carry out espionage and assassinations of key enemy figures, and over the course of 300 years successfully killed two caliphs, and many viziers, sultans and Crusader leaders. Under leadership of Imam Rukn-ud-Din Khurshah, the Nizari state declined internally, and was eventually destroyed as the Imam surrendered the castles to the invading Mongols. Sources on the history and thought of the Ismailis in this period are therefore lacking and the majority extant are written by their detractors. Long after their near-eradication, mentions of Assassins were preserved within European sources such as the writings of Marco Polo, where they are depicted as trained killers, responsible for the systematic elimination of opposing figures. Ever since, the word "assassin" has been used to describe a hired or professional killer, paving the way for the related term "assassination", which denotes any action involving murder of a high-profile target for political reasons.