In 1257, Niccolò and his brother Maffeo Polo were invited to stay at the Assassin Order's fortress of Masyaf in Syria. Having been visited by Altaïr Ibn-La'Ahad's son Darim in their home in Constantinople, they journeyed to Masyaf, where Niccolò had various discussions with Altaïr about the latter's life. As a result, Niccolò wrote these stories down in a journal, which ended up covering all of the Assassin's key moments. The journal also acted as a sort of diary, giving details of the Polo brothers' actions during their stay in the village.
Niccolò intended to pass the journal to his son, Marco. In 1495, the journal fell into the hands of the Spanish Inquisition. Not understand its true value, they were prepared to trade it to agents of the Byzantine Templars. Through a Templar turncoat, Aleksei Zima, the Ottoman Brotherhood of Assassins learned of these plans. He, along with the current Ottoman Mentor Ishak Pasha and future Assassin leader Yusuf Tazim, traveled to Spain and cooperated with the Spanish Brotherhood of Assassins to thwart the deal. However the trade, which had actually already taken place, was a trap and the Assassins were forced to escape from an ambush.
By the 16th century, the journal had come in the possession of the Ottoman prince Cem - a Templar - who notified his nephew Ahmet of its existence. Ahmet passed it to Leandros, a Byzantine captain who headed an expedition to open the library of Altaïr Ibn-La'Ahad hidden beneath Masyaf's fortress.
The Mentor of the Italian Assassins, Ezio Auditore da Firenze, arrived in Masyaf in 1511, on a quest to enter the library as well. Learning from a common worker forced into serving the Templars that Leandros had the journal in his possession, Ezio sought Leandros out, and, after a long chase through Masyaf and a nearby village, the Assassin was able to kill Leandros and obtain the journal.
Learning that the keys needed to open Altaïr's library were located in Constantinople after reading the journal, Ezio set out on a journey to the city, while continuing to read the journal along the way.