The Hashshashins (where we get our word "assassins") were active during the Abbasid Era of the Arab Period of Hegemony within the Islamic Period of Hegemony. The Hashshashins were Nizari Ismaili Shiite Muslims. They were led by Hassan-i-Sabah who, through the use of hashish, gave his recruits the impression that he was God and he wanted them to do his will. Until the coming of the Mongols under Hulagu Khan, the Hashshashins were very good at their job and they assassinated many high-profile people.
Though they are one of the most famous assassin groups, they are by no means the only ones. Others include Al Qaeda, the Mafia, the Black Hand, the IRA. The CIA, for example, is reported to have made 638 attempts to remove Castro from power.
For more information, read:
Military History Magazine (October 2002): Perspectives
Al Qaeda Training Manual
638 Ways to Kill CastroMilitary History Podcast is sponsored by Armchair General Magazine and the International Research and Publishing Corporation