Yes, the Assassin's Brotherhood of Assassin's Creed is based on the Hashshashin (early Arabic form of the word "assassin") a real life sect of exiled Ismaili Muslims formed by Hassan i-Sabbah that existed during the Crusades in medieval Syria and Persia.They were notorious for their stealth tactics used to eliminate key rival figures and balance the power between their enemies.
Though their primary enemy in the game are Crusaders, most of the victims of the real assassin order were Sunni Muslims. In fact, it's believed that Richard the Lionheart paid the assassins to kill Conrad de Monferrat, King of Jerusalem.
Masyaf, the "home" of the brotherhood in the game, is a real place in Syria, though its appearance differs from the video game. Masyaf is an old Byzantine castle that was refortified by Hashshashin leader Rashid ad-Din Sinan and his assassin order and was their capitol from the 12th to 13th centuries.
The Hashshashin were highly skilled killers. It is said that an assassin once sneaked into the heavily guarded tent of the Sultan Saladin in the middle of the night, during his siege on Masyaf in 1176. The assassin left a note hung by a dagger, that Saladin would be killed if he did not withdraw. Saladin feared for his life and ordered a truce with the assassins.
The Hashshashin fell around 1250 when the Mongols descended on the assassin stronghold of Alamut. The Hashshashin held off but a new assassin leader, Rukn al-Din, surrendered to the Mongols and ordered all other Hashshashin castles to follow. Alamut contained a grand library of the order's history as well as rare relics, but was destroyed when the Mongols took the city, and the assassin order with it.
Some of these real life assassins escaped the siege and became hired killers.
Altair in the game is not known to be an actual person in history, but represents their culture. The character Altair is said to be born 11 years before the real Saladin's siege on Masyaf. Al-Mualim is loosely based on Rashid ad-Din Sinan. Many figures in the game are real people from history.
