The fall of Masyaf was an event during the mid 13th century in which the grandson of Genghis Khan, Hülegü Khan, led an assault on the Assassin fortress of Masyaf in an attempt to annihilate the Order for their part in his grandfather's death, in 1227.
The attack caused the ultimate disbandment of the Assassins residing in Masyaf, which was completely deserted before the Mongols could claim their victory. Prior to the siege, the majority of the Assassin strongholds east of the Levant had already been destroyed.
To stop the major threat of the Mongols, who were making an unstoppable march from the East, Altaïr Ibn-La'Ahad, his wife Maria, son Darim and Qulan Gal traveled to the Xia province in Mongolia, in order to infiltrate a Mongolian camp where Genghis Khan was supposedly located. There, Altaïr, Darim, and Qulan snuck into the camp with the aim of assassinating Khan, however accidentally raised the alarm, resulting in Khan fleeing the camp on horseback. Khan was then hit by an arrow from Qulan's crossbow, causing him to fall from his horse. Moments later, Darim finished Khan with an arrow from his own crossbow.
Though most of the Assassins left the fortress to increase their numbers throughout the world, enough remained to hold off the Mongolians temporarily. During the assault, the Mongols attacked the walls of the village and fought on the ramparts, before being forced to retreat. During the battle, Altaïr had the two Polo brothers meet him in the main courtyard of the fortress. Once there, Altaïr passed on his life's work to the Polos; his Codex.
Altaïr then escorted the two brothers out of Masyaf, using the Apple of Eden to fend off the Mongols before giving the Polos his Memory Seals, which also acted as the Keys to his underground library, instructing them to hide each of them if necessary. As the Polo brothers left Masyaf, Altaïr led the remaining Assassins to fight the Mongolians.
With the Polo brothers able to escape successfully, Altaïr had the remaining Assassins and villagers of Masyaf evacuated, before the main Mongol army arrived to destroy the citadel. However, Altaïr stayed behind in Masyaf, intending to seal himself inside the library. Before entering the library, Altaïr told Darim that the library must stay hidden at all costs, and that he should leave before the Mongols returned. After embracing his son and bidding him farewell, Altaïr then entered his library and sealed away the Apple.
Altaïr subsequently imprinted this final memory on his sixth and last seal, before finally passing away at the age of 92.
After the Mongolian attack, Masyaf was completely abandoned by the Assassins. Instead, the Assassins began to operate universally, from a series of Guilds based across the known world.
Having fled from Masyaf, the Polos made their way back to Italy, first stopping in Constantinople to accomplish their task in concealing the Memory Seals. Two days into their journey, a Mongol attacking party had stolen the Codex from the Polo brothers, leaving them with only the Seals.
Guided by their knowledge and the importance of their task, Maffeo and Niccolò Polo finally arrived in Constantinople, hiding the Seals throughout the city to guarantee their secrecy. The two brothers had also set up a trading post, somewhere northwest of Hagia Sophia, and an Assassin Guild, writing the end of their journal on the day they hid away the final Seal.
After Hülegü Khan had effectively destroyed the Levantine Assassins, he continued his campaign throughout the Middle East for two more years. However, he was eventually hunted down and assassinated by the Mongolian Assassin Nergüi in 1265, in retribution for Masyaf's abandonment and the torture he had received at the hands of the Khan's army while he had been an Apprentice.