The Assassins (from Persian: حشیشیان Arabic: حشّاشين Ḥashshāshīn) is the name used to refer to the medieval Nizari Ismailis, particularly those of Persia and Syria, by Westerners. Often characterized as a secret order of assassins 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 the Persian territories by capturing and inhabiting several mountain fortresses under the leadership of Hassan-i Sabbah, who is typically regarded as the founder of the Assassins. While ""Assassins"" typically refers to the entire medieval Nizari sect, in fact only a class of acolytes known as the fidai actually engaged in assassination work. Lacking their own army, the Nizari relied on these trained warriors to carry out espionage and assassinations. The Nizari were feared by the Crusaders, who referred to them collectively as Assassins. The Crusader stories of the Assassins were further embellished by Marco Polo. 19th-century European orientalist historians such as Joseph von Hammer-Purgstall also referred to the Nizari collectively as Assassins and tended to write works about them based on biased accounts by medieval Sunni Arab authors, which they often took at face value.The name ""Assassin"" is often said to derive from the Arabic word Hashishin or ""users of hashish"", a misnomer thought to have been derogatory and used by their adversaries during the Middle Ages. Originally applied to the Nizari Ismaelis by the Mustali Ismailis during the fall of the decaying Ismaili Fatimid Empire and the separation of the two streams, it is possible that the term hashishiyya or hashishi in Muslim sources was used metaphorically in its abusive sense (i.e. ""social outcasts"", ""low-class rabble"", etc.), while the literal interpretation of this term in referring to the Nizaris (as hashish consuming intoxicated assassins) may be rooted in the fantasies of medieval Westerners.Long after their near-eradication at the hands of the Mongol Empire, 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.