Although known as early as the 8th century, the foundation of the Assassins is usually marked as 1090 when Hasan-i Sabbah[?], established his stronghold in the mountains south of the Caspian Sea at Alamut. A Yemeni emigrant and an Ismaili Shiite, Hasan set the aim of the Assassins to destroy the power of the Abbasid Caliphate[?] by murdering its most powerful members. Hasan ibn Sabbah was also known as "The Old Man of the Mountain", however, this is likely to have been a mistake in translation, since "Old Man" is the literal translation of "Sheik". Much of the current western lore surrounding the Assassins stems from Marco Polo's supposed visit to Alamut in 1273, which is widely considered mythical (especially as the stronghold was allegedly destroyed by the Mongols already in 1256).
The group inspired a aura of fear out of all proportion to their power. Legends stated that the Assassins were trained used ideology and drugs to believe they were assured a place in paradise if they were successful in murder with their golden daggers. They subdued, kidnapped, drugged and seduced the fiercest caravan guards, convincing them with elaborate means that they had died and awakened in Paradise. Thereafter, on subsequent raids, they fought furiously, believing that their death would only return them to that Paradise. The training technique was sophisticated for its time, especially the use of Ismaeeli dogma, drugs, and sex in contradictory combinations.
However, the "hashshashin" (often anglicized "hashishim") name was likely given to the Assassins by their enemies, and there is no evidence that the Assassins ever used drugs for this or any other purpose.
Most of the victims of the Assassins were Sunni Muslims. There were some extremely highly placed victims including Nizam-ul-Mulk[?]. Christians were largely untouched by the depredations of the Assassins, it was not until the middle of the 12th century that they had even really heard of them, although Conrad of Montferrat - the King of Jerusalem was a victim (the Assassins may have even been hired by Richard the Lionheart).
The power of the Hashshashin was destroyed by the Mongols as Alamut was destroyed in 1256 by Hulegu Khan[?], but several smaller sects remain to this day. During the Mongol assault, the library of the sect was destroyed and that is why we don't know so much about them.
The assassins figure in several conspiracy theories.