A short time after the "Little Judgment Day," and after the failed attack on the Assassin headquarters in Galata, Hasan received a letter from Prince Ahmet, telling him about the Prince's uncle, Cem, who he mentioned as the one responsible for starting the Ottoman and Byzantine Templar goal.
Doing as he was instructed, Hasan soon found the Memory Seal located there, with Ahmet further elaborating on Cem's goals and his dealings with the Borgia on his return. Ahmet also confided in Hasan that Manuel Palaiologos had sent Ahmet an invitation to meet him, and that Manuel claimed it was he who was responsible for the influx of mercenaries in Constantinople.
Additionally, Ahmet told Hasan that while he could not call Manuel a friend, nor could he trust him, Manuel was a former associate of Cem and that they "must not let such a monumental mistrust lead to the misfortunes that befell Cem," and that Ahmet and Manuel together could unite the Western and Eastern worlds into one Empire.
With this in mind, Ahmet gave Hasan instructions to arrange a meeting with Manuel, though while he was on his way to meet with the Templar, Hasan was attacked by Assassins, but he eventually came away as the victor.
Following this, Hasan attented the meeting between Ahmet and Manuel, and claimed that while the two seemed an unlikely pair, they had grown to "trust each other in the past few days," and that each established their roles; Ahmet would search for the Memory Seals, while Manuel trained new Templars using established bases in Rhodes, Derinkuyu and northern Thrace.
Hasan was also with Ahmet when the latter asked Manuel to tell him more about Cem, and so he heard Cem's story. However, the conversation was cut short when a detachment of Selim's men found them, with Hasan taking on a role in rallying the defenders and successfully beating the detachment.