from Wikipedia:

A future assassin was subjected to rites very similar to those of other mystery cults in which the subject was made to believe that he was in imminent danger of death. But the twist of the assassins was that they drugged the person to simulate a “dying” to later have them awaken in a garden flowing with wine and served a sumptuous feast by virgins. The supplicant was then convinced he was in Heaven and that Sabbah was a representative of the divinity and that all of his orders should be followed, even to death. This legend derives from Marco Polo, who visited Alamut just after it fell to the Mongols in the thirteenth century.

Other accounts of the indoctrination attest that the future assassins were brought to Alamut at a young age and, while they matured, inhabited the aforementioned paradisaical gardens and were kept drugged with hashish; as in the previous version, Hassan occupied this garden as a divine emissary. At a certain point (when their initiation could be said to have begun) the drug was withdrawn from them, and they were removed from the gardens and flung into a dungeon. There they were informed that, if they wished to return to the paradise they had so recently enjoyed it would be at Sabbah’s discretion, and that they must therefore follow his directions exactly, up to and including murder and self-sacrifice.