The Yezda were nomadic tribes, fighting almost exclusively as mounted archers. In battle they drew up by clans, rather than the ordered ranks of the Makuraner lancers or Videssian troops.
At some point prior to the invasion of Makuran, Avshar came among the Yezda, and presumably through demonstration of his magical and war prowess turned the Yezda to the worship of Skotos. When Avshar used a demon to slay Varahran, the last Makuraner King of Kings, the Yezda invaded the nation, occupying it and instituting the cult of Skotos as the state religion. The religion involved human sacrifice and its priests wore robes the color of dried blood.
Under the Yezda, the indigenous Makuraners and the farmers of the Land of the Thousand Cities suffered from wanton violence and pillaging.
At the time of the arrival of the Roman Legion, Wulghash was Khagan of the Yezda, but Avshar wielded significant power and influence throughout the kingdom, and his battle magic made him a figure of awe to many of the nomadic warriors. Many Yezda, including the Minister of Justice Tabari, were secretly traitors to Wulghash, and loyal to Avshar.
At the Battle of Maragha the army of Yezd routed the Videssian army, and Avshar slew Mavrikios Gavras, Avtokrator of Videssos, in single combat. Many tribes and clans of the Yezda moved into the Videssian westlands as a result of this defeat, where they took up herding. These Yezda defeated the forces of the rebel Namdalener Count Drax, and negotiated a hostage exchange with Marcus Scaurus.
Avshar usurped the throne from Wulghash while Scaurus and Gaius Philippus were guests of the Khagan in the palace at Mashiz. Wulghash escaped by using magic to give his appearance to one of his guards, and later met Scaurus and Phillippus in the catacombs beneath the palace. Wulghash rode off to gather loyal troops, arriving at the final battle in time to defeat the pro-Avshar faction and tip the balance in favor of the Videssians. At the end of the series Wulghash restored his authority as Khagan.
After the defeat of Avshar with the Druidic enchantments, Wulghash made peace with Thorisin Gavras, though sources indicate that the relationship between Videssos and Yezd remains fairly hostile. The tribes of Yezda in the Videssian westlands were disavowed by Wulghash.
Yezd is a city in Iran.
Since Makuran modeled on Sassanid Persia, it is possible that the Yezda are based on the Islamic tribes that invaded that country. It is also possible that the Yezda are an analog of the Seljuq Turks, as the Gavrai parallel the real-life Komnenoi dynasty, which ruled the Byzantine Empire during its wars against the Seljuqs.