Assassin's Creed used elements of the novel as inspiration for its story, but is not an adaptation of the novel. The novel doesn't feature a machine that sends people into the bodies of their distant ancestors. Nor does it feature a conspiracy by a group of historical figures that are secretly working for the Knight's Templar. The novel also involves different characters (Al Mualim is not based upon Hassan ibn Sabbah, but Rashid ad-Din Sinan) and different locations (Arsuf, not Alamut), and a different time period (1192, not 1092). In fact, other than Al Mualim ordering people to jump to their deaths at the start of the game (which is later revealed to be a hoax to scare their enemies), and the fact that both are about the Hashshashin, there is hardly any similarity at all between novel and game.