The Isma'ilis are an Islamic sect which dates back to the early days of Islam. There are somewhere between 4 and 20 million Isma'ilis today. They do not constitute a majority in any country. They develop as an offshoot of Shi'a, out of a disagreement over who was the next imam.
In the early 11th century, al-Hassan ibn-al-Sabbah became head of the Persian Nizari sect of Isma'ilis. He established a mountain stronghold in northern Persia, and became known as 'The Old Man of the Mountain'.
According to legends, Al-Hassan trained young men to be assassins and sent them out into the world on suicide missions. To inspire them, he would drug a young man into unconsciousness, and when he awoke he would find himself in a garden filled with beautiful girls who told him he was in Paradise. After spending a few hours of blis with these girls, and enjoying other delights of the garden, he would be drugged again. When he awoke, he would be back in the familiar world, with only a memory of 'Paradise'. The young man would then be told he would return to Paradise only if he died in the service of Al-Hassan. The young men so tricked into service became one of Islam's earliest terrorist gang.
This legend was introduced into the European consciousness during the Crusades. Marco Polo reported on it in his widely-read narrative, though he was reporting 150 years after the fact. How much truth there is to the legend is a matter of scholarly debate. The word 'assasin' derives from a name of al-Hassan's sect, which meant 'hashish-eater' in Arabic. It is possible that the Nizaris got this name from using hashish; but it is also possible that the name was an insult used by their opponents. The Nizaris were basically eliminated when they made the mistake of assassinating a relative of Genghis Khan.