In 1191, at the time of the Third Crusade, the Assassin Altaïr Ibn-La'Ahad killed several influential individuals, both Crusader and Saracen alike, who were secretly members of the Templar Order. Some of these were located in the Syrian capital of Damascus, and ultimately, their deaths caused the Templars to flee to the island of Cyprus. After each assassination that the head of the Levantine Assassins, Al Mualim, ordered him to carry out, he would return to the Assassin fortress located in the village of Masyaf.
In 1511, Ezio Auditore da Firenze arrived at the city of Masyaf to find it abandoned. In the fortress however, he was ambushed by Templars, where he was greatly outnumbered, injured and captured, as well as having one of his Hidden Blades destroyed. After escaping from a death by hanging, Ezio found the gate to Altaïr's library, and after assassinating Leandros and acquiring a journal from his body, he learned that the Masyaf Keys were hidden in Constantinople, where he traveled afterward to find them.
Upon finding every key, Ezio returned to Masyaf with Sofia Sartor and entered the library, though he didn't find any books, much to his surprise. Instead, he found the corpse of Altaïr, holding a final key that contained his last memory: sealing himself and an Apple of Eden in the library, and his subsequent death.