"They are men sworn to a brotherhood, ... sworn to serve the Ghost Walker, if he should come. They give a finger to their God, in witness; and they live apart from the tribes, in little groups together. For centuries now, they have lived with each other and their vows, and nothing more. Now the Ghost Walker has come again, he stands among them, and they are trying to slay him..."
The Assassins - Terrorists of the Eleventh Century
Assassins - sect inside Shi'i Islam, more specifically the Nizari Isma'ilis, in the period from the 11th to the 13th century. Most significantly, the Assassins are famous for their fearless murders especially during the times of the Crusades.
Continued... There are many legends connected with the Assassins, and many are of dubious origin, more a result of medieval European story tellers' fantasy than facts (among these story tellers is Marco Polo). The main theme is that the Assassins performed their deeds under strong intoxication from hashish, resulting in their own deaths, but with the promise of immediate entry to paradise — a paradise that had been staged for them during their training at the Alamut, a paradise of sweet food and wine and beautiful and willing women.

These stories have never been confirmed by any investigations of contemporary Isma'ili sources, and there is good reason to believe that such a shortcoming is a clear indication that such stories are fabrications.

From the original sources, we learn that the Assassins changed the original Isma'ili doctrine, so that terrorism became a religious duty. Growing out from their centre in Kazvin, the Assassins built a number of strongholds all over Iran and Iraq.

The idea of a paradise constructed around Alamut was probably based upon the sayings of imam al-Kahir, where he talks about a Paradise that man has already entered. However, al-Kahir's paradise was meant as a spiritual one.

They had a system of terrorists as well as secret agents positioned in enemy camps and cities. The Assassins often worked closely with certain leaders of Muslim states, as their services were attractive: no one else was better able to kill important persons in enemy states. They were for long periods allied with the Christian crusaders, not because the Christians sympathized with them, but because they had common enemies.

One of the most important Muslim allies of the Assassins was the Seljuq ruler of Aleppo, Ridwan. Through this cooperation, they were able to establish themselves in the Syrian mountains, where several fortresses were erected. Without being recognized in the same way as other temporary states, the Assassins had in reality, their own state here. But the Assassins's influence over Aleppo came to be immense, and they effectively ruled the politics and economy of the city and its surroundings for a couple of decades.

Even Saladin came to treat the Assassins as allies, although he intended initially to eliminate them. The reason for this alliance, was that Saladin, following two assassination attempts, feared for his own life, and had more imminent enemies.

The Assassins were ranked according to intelligence, courage and trustworthiness. They underwent intense education as well as physical training.

When the Assassins were out on mission, they generally worked alone. Rarely did two or more of them work together. They dressed up as tradesmen or ascetic religious men, and spent a good deal of time in a city, in order to get well-acquainted with the houses and streets, as well as the daily routines of the future victim. The actual murder was performed with a dagger and in public, often inside the mosque on a Friday. By doing it all in public, the information about the deed was soon well known, and people were frightened. In general, the Assassin murderer himself was killed immediately thereafter by guards of the victim.