All right, I'm going to skip over levels 10-14 until such time as I can find a reason to chat about the terms Expert, Senior, Chief, Prime or Guildmaster as they apply to Assassinry. So hold on to your dice bags kiddies 'cuz we're going straight to the top: Grandfather of Assassins.
As was mentioned previously, the word assassins was first used to describe a fanatical cult in Persia from the middle ages. The founder of this cult was Hassan-i-Sabbah, who, by shacking up in his mountain castle, earned the title Sheik al Jebal which translates to "Prince of the mountain" or, the preferred term in this case, "Old Man of the Mountain."* This old man was the original Grandfather of Assassins; a single person who ruled over the entire order of death bringers. But unlike the D&D usage, though eh is responsible for numerous murders, he was not a professional killer and very likely he never killed anyone with his own hand. Rather, he was the dude with the charisma to charm you into devoting your life to the cause, and also the dude to tell you when the cause needed you to take action and whack some mo' fo' who'd talked trash about your cult.
* Alternatively, there are sources that say that it was the leader of the Syrian branch who was referred to as the Old Man of the Mountain.
This dude is also, allegedly, the grandfather of the mystic cult phenomenon; the Crusaders, and particularly the Knights Templar, took what they learned about the Order of Assassins back to Europe and thus was born the culty trappings we now associate with mystic groups still extant such as the Freemasons, Illuminati, Rosicrucians, and the American Institute of Architects.
For those interested in further reading, there's a novel titled Alamut written by the Slovenian author Vladimir Bartol that is based on the cult of Assassins and the treachery of Hassan-i-Sabbah. Published in 1938, the book was translated into 18 languages and has achieved bestseller status in Spain and France, but it was not translated into English until 2004. A few pages into it and I wish they had taken a few more years to work on that translation. One gets the sense that the poor fellow approached the translation as if he were transcribing legal documents, not reconstructing linguistic subtleties to convey a narrative... or whatever. The result is repetitive use of simplistic sentence structures which make for very tedious reading. Despite its subject matter, it seems like it was written for children. Brush up on your Spanish or French--or, better yet, your Slovenian--if you really wanna read this one.