Hashishiyyin:

The Assassins, or hashishiyah, were a medieval Shiite sect known as the Nizari Ismailis. They were primarily in Syria and Persia (present day Iran).

Hashishiya means hashish user, and the name comes from the lore that followers of the group drugged themselves before carrying out assassinations. No one knows if this is actually true.

A translation of hashish-users, Assassins, entered Western language with the Crusader tales. According to orientalist Bernard Lewis, the Italians were already using "Assassin" generically in the 14th century to mean "professional murderer."

Leader:

Hassan Sabbah.

Sabbah was an adherent of the Nizari Ismaili Shiite sect, whose members lived in Persia and Syria from the 11th century until the destruction of their dynasty by Mongols in the 13th century. Sabbah, eager to return Persia to Shii hands, committed himself to helping achieve the transformation by force. In 1090, he seized territory in northern Persia (today, Iran), an area that was then ruled by Sunni Saljuq Turks.

Organization:

Under Sabbah's direction, an organization developed that could challenge the Saljuqs. Lebanese novelist Amin Maalouf describes an organization that today would quickly be labeled terrorist:

All members, from novices to the grand master, were ranked to their level of knowledge, reliability and courage. They underwent intensive training courses of indoctrination as well as physical training. Hasan's favourite technique fng terror among his enemies was murder. The members of the sect were sent individually - or more rarely, in small groups of two or three - on assignments to kill some chosen personality.

They generally disguised themselves as merchants or ascetics and moved around in the city where the crime was to be perpetrated, familiarising themselves with the habits of their victims. Then, once their plan was ready, they struck. Although the preparation was always conducted in the utmost secrecy, the execution has to take place in the public, indeed before the largest crowd. That was why the preferred site was a mosque, the favourite day Friday, generally at noon.

For Hasan, murder was not merely a means of disposing an enemy, but was intended primarily as a twofold lesson for the public: first, the punishment of the victim, and, second, the heroic sacrifice of the executioner, who was called fidâ'î (plural : fidâ'în, or fedayeen), or 'suicide commando', because he was almost always cut down on the spot. (From A History of the Assassins

The Legend of the Asssassins:

Crusaders, who were camped out in areas around Jerusalem to protect Christian pilgrimage routes, returned to Europe with fantastic tales of the assassins as stealthy political murderers following their cult leader, Sabbah. In Western tales, Sabbah was known as the Old Man of the Mountain.

One of the most famous accounts was one told by Marco Polo, which one scholar has dismissed as a "fully embellished and utterly fanciful account" in which "the Nizaries were basically depicted as a drug-crazed and secret order of assassins driven to senseless murder by their mischievous Old Man." (Farhad Daftary, Journal of the American Oriental Society, 122:3. 2002)

The Assassins were considered so powerful that they were believed to be behind a number of political murders in Europe, though they never were.

The Murder of Conrad de Montferrat, King of Jersualem:

One of the most well-known political murders attributed to the assassins was that the Conrad de Montferrat. Conrad I of Jersusalem, as he was also known, was an Italian military commander in the Holy Roman Empire. After a short stint working for the Byzantine emperor, Montferrat set off for Syria/ Palestine to join the Crusaders in their battle against Saladin's Muslim fighters.

Two days before being crowned King of Jersualem, Conrad was stabbed in the side and back by two Assassins disguised as monks while walking to his palace. He died shortly afterward. Both Assassins were captured and tortured on the rack, where one died and a second confessed that Richard I of England had ordered the assassination. The probability that this is so was given credence in contemporary Arab and Western accounts, but there has never been a definitive answer to Conrad's assassination.

Ironically, the Assassins were sometimes allies of the Christian army, since they shared a common enemy, the Seljuq Turks.