Videssos is a series of fantasy novels series by Harry Turtledove.
"Videssos" can mean either the Empire of Videssos, or its capital city. The Videssian Empire is very similar to the Byzantine Empire, and much of its history and geography is analogous to Byzantine history. The main differences are the religion, the existence of magic as a usable force, and some details of the history of surrounding states. These states strongly resemble neighbours of Byzantine Empire - for instance Makuran is Sassanid Persia and Vaspurakan is Armenia (Vaspurakan is in reality a historic Armenian province). The map shown of the known world in the time of the various Videssos stories is a very loose east-for-west flip-flop of the Mediterranean.
Like Byzantium, Videssos is an absolute monarchy, ruled by an Emperor. The Emperor is assisted by a numerous, influential bureaucracy, which is powerful enough to have been able to enforce its choice for the throne, even in the face of military opposition; the bureaucracy and military are traditionally rivals for power and influence. Even Videssian offices and titles are the same as the Byzantine ones (autokrator, sebastokrator, vestiarios, drungarios).
The Videssian emperors and their history strongly resemble the Byzantine emperors.
The Videssian Empire is rather feudal, with powerful landowners wielding great influence. The class of yeoman farmers, who provided much of the best recruits for the military, is dwindling, analogously to what happened at several stages of Byzantine history.
Everything within Videssian life centers around religious beliefs. There are varied types of beliefs from pantheons of gods, spirit/demon worship, and dualist belief. The most widespread belief is in the dualist faith of Phos, the god of light, and Skotos, the dark god. The core of this faith is that Phos and Skotos will battle and only one will emerge. Even in this belief there is nuanced variations that lead to enmity between different countries.