Existing throughout two different empires, the Turkish Assassins were mainly involved in a struggle against the Byzantines, the Eastern European embodiment of the Templars. Even after the Byzantine Empire was dissolved, the Byzantines posed a threat to the Assassins, as they were determined to revive their fallen empire. Until the ultimate fall of the House of Palaiologos, the Assassins kept battling their Byzantine rivals.
In 1204, the leader of the Levantine Assassins, Altaïr Ibn-La'Ahad, journeyed to Constantinople in the Byzantine Empire - modern-day Turkey - hoping to introduce the Assassin Order there. However, due to the chaos caused by the ongoing Fourth Crusade, Altaïr was unable to recruit potential Assassins. In 1257, the tradesmen Niccolò and Maffeo Polo were invited to stay at the Assassins' fortress of Masyaf by Darim Ibn-La'Ahad, the son of Altaïr. Being trained into becoming Assassins themselves, Altaïr entrusted them with creating various Assassins Guilds abroad, as well as guarding Altaïr's Codex. When the Mongols eventually attacked, the Polo brothers left Masyaf, also being entrusted with safeguarding the five keys to Altaïr's library underneath Masyaf.
The Polos then traveled back to their home in Constantinople, where they were successful in creating a guild and bringing the Assassin ideals to the Byzantine Empire. However, the Byzantine Empire was controlled by the Templars; the last Byzantine Emperor, Constantine XI, ordered the capture and execution of Assassins throughout his Empire. It is unknown if his predecessors did the same.
The Byzantine Empire was eventually defeated and usurped by the Ottomans in 1453, resulting in the foundation of the Ottoman Empire. Templar presence in the Turkish region became minimal, allowing the Assassin Order to develop.
By the later half of the 15th century, the Ottoman Vizier Ishak Pasha - secretly an Assassin - took a leading role over the Turkish Assassins. Ishak Pasha fought and defeated Vlad "the Impaler" Tepes, a secret Templar, and brokered a truce between the Assassins and Ottomans.
During the late 1480s, Ishak Pasha had begun recruiting Ottoman citizens into the Assassin Brotherhood. One of these Ottomans was Yusuf Tazim, whose father also was an Assassin. In 1497, Ishak Pasha died. Control over the Turkish Assassins later shifted to Yusuf Tazim, who had by then become a Master Assassin. For unknown reasons, the Turkish Assassins seemed not to have any influence in Constantinople. In 1501, Ezio sent some of his own recruits to help set up a guild there. A year later, a group of Turkish Assassins, including Tazim, and Venetian Assassins met in Greece to make plans to end the Venetian-Ottoman war, being successful in doing so in 1503. The Assassins had realized that the Templars were spreading their reach to the New World. To ensure that they could keep their activities in check, the Assassins wanted maps to the Americas. They broke into the workshop of the famous Ottoman admiral and cartographer and Piri Reis, an exceptional Ottoman cartographer, who had been called away for naval duty. They found maps drawn in excellent detail, and found that he had tracked their common enemy to the New World. The Assassins boarded these voyages to follow.
Meanwhile, the Inquisition in Portugal and Spain caused several men and women to leave the country. This community flourished with scientists, artists and philosophers. Sultan Bayezid II welcomed these refugees, as he knew that they would strengthen his empire. King Ferdinand had planted spies in this community, and made sure they exposed nothing about themselves. However, Italian and Ottoman Assassins were watching for suspicious behavior and caught about a dozen spies, killed them and replaced them with their own men. These Assassins worked to strengthen the guild in Constantinople.
In 1506, after some initial conflict, the Ottoman admiral, geographer and cartographer Piri Reis - tired of the artificial boundaries that separate nations - joined the Assassin Order as a scholar and technician, eventually becoming a Master Assassin specializing in bomb crafting.
Following the earthquake in Constantinople in 1509, Templar remnants of the Byzantine Empire returned to the city, hoping to restore their fallen empire. The Assassins became engaged in a direct conflict with the Byzantines in the following years, though they also used the hatred between the Byzantines and Ottomans to their advantage, often purposely creating conflict between these factions.
In 1511, the leader of the Italian Assassins and a Mentor in the Assassin Order, Ezio Auditore da Firenze, journeyed to the Turkish Assassins' headquarters in Constantinople with the intention of finding the five keys needed to open Altaïr's library underneath Masyaf fortress. Greeted by Yusuf Tazim, Ezio was quickly taken in and introduced to various techniques that the Turkish Assassins used, including bombs and Hookblades. Though Yusuf still led the Turkish Assassins, it was Ezio who took a leading role over the Turkish Assassins against the Byzantine Templars. Recruiting many new Assassins and taking back Constantinople from Byzantine control, Ezio even sent Turkish Assassins to various regions across the Mediterranean, thwarting the Templars' plans and taking over cities from Templar control.
In late 1511, Yusuf led a group of Assassins to attend a cultural exposition held in Topkapı Palace by Ottoman Prince Suleiman, knowing that the Templars were going to infiltrate it. Joined by Ezio, the Assassins went disguised as minstrels from Italy, with Ezio distracting the guests and discovering the Templar infiltrators using his Eagle Sense. Successfully locating all Templars before they could bring harm to Suleiman, the Ottoman Prince and Ezio became friends and allies in the process. In early 1512, the Turkish Assassins aided Ezio Auditore in setting sail for Cappadocia to find the last Masyaf key, as the Janissaries were blocking the port to make sure Ezio did not leave the city. While destroyed the Great Chain spanned across the Golden Horn Ezio burned the Ottoman boats blocking the boat, Yusuf and a group of other Assassins kept the Ottoman guards at bay. Ezio successfully opened the harbor, and joined Piri Reis on the boat to Cappadocia.
Though before Ezio left, he asked Yusuf to protect a woman named Sofia Sartor, a bookseller Ezio expressed interest in. Yusuf agreed, and he and several other Assassins had taken up positions around Sofia's bookstore to ensure her safety. Though Ezio was able to eliminate Manuel Palaiologos in Cappadocia, marking the fall of the Palaiologi and the ultimate destruction of the Byzantine Empire, the Byzantine Templars became under the full control of Ottoman Prince Ahmet, the favored son of Sultan Bayezid II who had secretly made plans for the Templar Order from within the royal house. On order of Ahmet, the Templars attacked Sofia's store to capture her, knowing she was close to Ezio. Though managing to kill a large group of Templars, Yusuf was eventually stabbed fatally in his back. Ezio discovered the body upon his return to the city, which served as a large motivator for the Assassins to strike back at Ahmet and the Templars, who were hiding in the city's arsenal.
The Assassins led a full-on assault on the arsenal, and numerous Assassins and Templars faced off against each other. Ezio reached Ahmet deep inside the arsenal, but had to spare his life in order to find out Sofia's location. Offering a deal to trade Sofia for the Masyaf keys, Ahmet and the Templars left the arsenal.
Before going to the exchange near Galata Tower, Ezio appointed Tazim's lieutenant Dogan to become the new leader of the Turkish Assassins. Suspecting the exchange to be a trap, Ezio had the Assassins take up positions around the perimeter, and they jumped in as soon as Ezio's suspicions became true. Nonetheless, Ezio managed to successfully rescue Sofia, and the two of them chased after Ahmet in the countryside around Constantinople. After a long chase, Ezio and Ahmet ultimately found themselves falling from a cliff, only to be saved by Ezio using one of his parachutes. Right afterwards, they were joined by Ahmet's brother Selim, Suleiman's father, who had just become Sultan and was on his way back to Constantinople. Selim attacked and strangled Ahmet, and eventually shoved Ahmet off another cliff to his death. Only for his son's respect for the Assassins, Selim spared Ezio's life, but ordered him to permanently leave Constantinople.
Nonetheless, Ezio and Sofia returned to Constantinople, with Suleiman's knowledge and Selim's temporary consent. Ezio made preparations to leave and to stabilize the Assassins in the city. Azize, a female Assassin who had taken care of Sofia's bookshop in her absence, was allowed to run the shop full-time and the Brotherhood bought it from Sofia. With Dogan as the leader of the Turkish Assassins, Ezio knew that the Brotherhood in the Ottoman Empire would continue to thrive. Eventually, Ezio left for Rome with Sofia, who had become his fiancée.