Masyaf Key

The Seals of Altaïr were a collection of five artifacts originally possessed by Altaïr Ibn-La'Ahad that, when brought together, would unlock a powerful weapon hidden deep within the fortress of Masyaf; a weapon sought by both the Assassins and the Templars.

Ezio Auditore da Firenze, Grand Master of the Assassin Order learned of the seals' existence from a captive Templar during his exploration of the aged Assassin fortress, and traveled to Constantinople in the hope of obtaining them.

Source

 * Game Informer magazine, May 2011 edition