In the video game Assassin's Creed, the Order of Assassins battle the Knights Templar. The Knights Templar were real enough, but the Order of Assassins is fictional. However, they are based on tales of the medieval Nizari Ismailis, a breakaway sect of Shiite Muslims that flourished between the 11th and 13th centuries. They were so devout in their beliefs that they were willing to kill and to die to bring the Nizari to power, and keep them there. It got to the point where any political assassination in Egypt or Syria was ascribed to the Nizaris.
The popular image of the the Nizari Ismailis in the West began with the travelogues of Marco Polo, which may have come from Sunni sources, and other wild legends that had been filtered through retellings, often with political aims. The Crusaders had plenty to say about the assassins, but those tales were greatly exaggerated and embellished. Strangely, the fictionalized version of the sect used in the game Assassin's Creed is more in line with the historical Nizari than traditional Western accounts. Read about the real Nizari Ismailis at Smithsonian.