It is indeed recorded in history that during the Third Crusade (1189-1192), there was such an assassin organization, which was based in Masiaf Castle for many years.

The real League of Assassins was founded by Persian missionary and philosopher hassan sabah. In order to maintain his own order and interests, he will use assassins to get rid of some leaders and politicians, so as to shock his opponents. These assassins are well-read and have high martial arts skills, just like Artell in the game. They need to secretly collect information about the target before the action. Sometimes they don't kill people, they just make threats, such as leaving daggers on pillows. These practices are often enough to make people obey Sabah's will.