The original Assassins, also known by many variants such as Hashishin or Hashashiyyin, were an Islamic sect or cult at the time of the Crusades. Their practice of stealthy murder has given us the modern meaning of 'assassin'.
It is widely believed that their leader, Hassan-i-Sabah, known also as The Grand Master or The Old Man of the Mountain, controlled his followers by means of hashish or another drug, thereby tricking them into believing he could show them Paradise. There is, however, reason to suppose that this is is false. Firstly it arises from the unreliable tales of Marco Polo. Secondly, scholars say their name is most likely to derive from Asasiyun, meaning 'those who are faithful to the foundations [of the faith]'. And finally, while hashishim does indeed literally mean 'hashish-smokers', it was more usually used as a general-purpose shorthand for wickedness than as a specific accusation.
It seems more likely that the Assassin's headquarters in what is now Iran, Alamut or 'Eagles Nest', contained yet another entry-way into the world in which Teleleli lies, or instructions on creating one. This would explain both the story of 'showing Paradise', and the Assassin's otherwise inexplicable ability to appear as if from thin air. The fact that the library of Alamut was burned after its capture by the invading mongols suggests that it may have held books on travelling between worlds.