So the inspiration of this here string of posts on the AD&D Assassin class level titles is finally at hand; we've reached the eponymous 9th level: Assassin.
The word assassin is derived from the word Hashshashin, a term used in Syria--not the Lendore Isles as some would have you believe--to describe a fanatical sect of Islam called the Ismailis who were a small but troublesome religious/political force in the 11th-13th centuries. Lacking a powerful army and considered heretics by the Muslim establishment of the day, the Ismailis holed up in a few castles in an isolated valley in northern Persia and dissuaded their numerically superior enemies from invading by murdering key political figures. Though the cult was centered in the mountains of Persia, the term assassin is believed to have come about in reference to the Syrian branch of the group that the Crusaders came in contact with. Hashshashin means "outcast" or "rabble" but also was associated with users of the psychotropic herb hashish. And although most Assassin scholars believe that it is its meaning as a pejorative term for outcasts that originally inspired the application of the word, the western world has latched onto that hashish connection and had a blast with it ever since.
Anyway, the cult was formed by a man named Hassan-i-Sabbah, among other spellings, who took control of an impregnable castle known as Alamut, and gathered around himself a throng of devoted followers who were willing to obey his every command. They were trained in the fine arts of duplicity and sent out into the world to gain the trust of various persons of interest to the Assassins, usually folks with titles like Sultan, Vizier, or Imam. Once they'd inveigled their way into the homes of these dudes as household servants, body guards, or even trusted advisers, they lived a life of faithful service to them, never betraying their origins until such day that the signal came from Alamut, at which point they would cram a dagger through their erstwhile compadre's ribs. They often kept up their ruse for years or even decades and, depending on how things worked out, the signal might never come. Deep cover, my friends. Indeed, there is very early usage of the word assassin that implies not a hired killer but a person of unsurpassed devotion.
Once the devotee had done the deed, the assassin did not generally slink away into the night; no, getting caught was, more often than not, part of the job. By submitting to capture and execution, the assassins did more than just kill the leaders of their enemies; the combination of their skill at subterfuge and complete devotion wreaked havoc on their enemies psyches as well. A culture of paranoia spread like wildfire, and speaking out--much less taking military action--against the Assassins soon fell into disfavor.
Most of the useful information in this post is derived from the book The Assassins by Bernard Lewis, originally published in 1968 by Basic Books, Inc. New York. It's a somewhat scholarly tome describing the religious and political events of Islamic world during the time of the Assassins. Also, the wikipedia entry for Hashshashin is pretty informative.