Masyaf is the Assassin headquarters in the original Assassin's Creed.
In the game, Masyaf is a mountain settlement in the Levant and the home of the Assassin Brotherhood. Altaïr returns there between missions to receive instructions, report to Al Mualim, and improve his equipment. The settlement is also presented as the Brotherhood's principal stronghold during the game's 1191 storyline.
The fictional game location is based on the real Syrian town of Masyaf, which has a prominent medieval castle. Historical sources associate the castle with the Nizari Ismaili community, often linked in popular culture with the “Assassins.” Ubisoft uses that history as inspiration while building a fictional version of the Brotherhood.
Masyaf should not be confused with Monteriggioni, the Italian villa that becomes an important base in Assassin's Creed II and Brotherhood. It is also different from the phrase “Requiescat in pace,” a recurring expression associated with Altaïr and Ezio rather than a place.