In Assassin's Creed Revelations, Altaïr Ibn-La'Ahad dies in 1257.
The game depicts the final stage of Altaïr's life through memory sequences discovered by Ezio Auditore. After returning to Masyaf and securing the Apple of Eden, Altaïr spends his later years preserving the knowledge and legacy of the Assassin Order. He eventually seals himself inside the Masyaf library with the Apple and dies there at the age of 92.
Altaïr's earlier adventures take place in 1191, the year associated with the first Assassin's Creed. That date is a common source of confusion because both games revisit his life, but it is not the year of his death. The 1257 date places his final moments in the era of the Mongol Empire's expansion and shortly before the late-thirteenth-century setting of Ezio's Constantinople story.
Revelations was released in 2011 and links Altaïr's memories with Ezio's search for Masyaf's keys.