Fall of Masyaf

The Fall of Masyaf was an event during the 13th century where the grandson of the leader of the Mongol Empire Genghis Khan led an assault on the Syrian Assassins' fortress of Masyaf in an attempt to annihilate the Assassins for their part in his grandfather's death in 1227.

The event caused the ultimate disbandment of the Assassins from Masyaf, which was completely deserted before the Mongols could claim their victory. Alongside Masyaf, most of the Assassins' fortresses in the Levant were already destroyed by the Mongols in 1256.

The event also caused Altaïr's Codex to wind up in the hands of the Mongols, until it was retrieved by Niccolò Polo, Maffeo Polo and Marco Polo.

Genghis Khan's assassination
To stop the major threat of the Mongols, who were unstoppably moving from East towards West, Altaïr, Maria, Darim and Qulan Gal headed to the Xia province in Mongolia, visiting a camp set up by Mongolians, in which Genghis Khan supposedly was. There, Altaïr and his son, together with Qulan, sneaked into the camp and accidentally raised the alarm for Genghis Khan, who then fled on horseback. A few moments later, Khan was hit by an arrow from Qulan's crossbow, and was then finished by Darim.

Assault on Masyaf
During 1257, the Mentor of the Syrian Assassins, a 92-year old Altaïr Ibn-La'Ahad, was interviewed by Niccolò Polo and his brother Maffeo Polo in order to document the stories of his life and the Order. Eventually the brothers needed to make their departure, as the Mongolians attacked unexpectedly and besieged the village.

Though most of the Assassins left the fortress to increase their numbers throughout the world, enough remained to hold off the Mongolians temporarily. Passing on his life's work: the Codex and Seals, Altaïr instructed the brothers to hide the seals throughout the city of Constantinople.

Before escaping, Niccolò told the Mentor to "take care of [him]self", to which he replied he would "consider it."

Aftermath
Having fled from Masyaf, the Polos were on their way back to Italy, stopping by Constantinople to first complete their task. Two days on their way to the city, a Mongol attacking party had stolen the Codex from the Polo brothers, leaving them only with the Seals. Guided by their knowledge and the importance of their task, Maffeo and Niccolò Polo arrived in Constantinople, hiding the Seals throughout the city as to make sure of their secrecy. The two brothers had also set up a trading post, somewhere northwest of Hagia Sophia, and an Assassins Guild, writing the end of their journal on the day they locked away the last Seal.

Source

 * Assassin's Creed: The Secret Crusade