The office of Emperor of the Roman Empire in the East (retroactively renamed Emperor of the Byzantine Empire) was never formally created, but instead gradually "spun off" from the of the original Roman Empire. In 330, Roman Emperor Constantine I moved his capital to Constantinople, the city formerly known as Byzantium, which remained the seat of power of the Eastern Empire after the schism with the Western subdivision. The Byzantine Emperors all claimed rulership of "the Romans" until Constantinople's fall to the Ottoman Empire in 1453 AD. The title of all Byzantine Emperors until the reign (610-641) of Herakleios, was officially "Augustus," although other titles such as Dominus were also used. Their names were preceded by Imperator Caesar and followed by Augustus. Following Herakleios the title commonly became the Greek Basileus (Βασιλεύς), which had formerly meant "king" but was then used in place of Augustus. Following the establishment of the rival Holy Roman Empire in Western Europe, the title "Autokrator" (Gr. Αὐτοκράτωρ) was increasingly used. In later centuries, the Emperor could be referred to by Western Christians as the "Emperor of the Greeks". Towards the end of the Empire, the standard imperial formula of the Byzantine ruler was "[Emperor's name] in Christ, Emperor and Autocrat of the Romans" (cf. Ῥωμαῖοι and Rûm).

Constantine XI Palaiologos was the last Christian Emperor of Constantinople, however the conquering Ottoman Emperors used "Caesar of the Romans" among their titles, up until their Empire fell in 1922.

Harry Turtledove's fiction has made the following uses of the office of the Byzantine Emperor.

Agent of Byzantium

Although the Roman Empire was torn by civil war at the turn of the 7th century, it endured and consolidated its status as one of the leading powers in the world by the early 14th century. Some Emperors had more colorful personalities than others.

While the ill-fated Maurice was Emperor, Mouamet, an Arabic merchant from the Empire's eastern fringes, converted from polytheism to Christianity, and later became a bishop and writer of hymns. While Saint Mouamet remained a popular cultural touchstone down through the ages, it is doubtful that his conversion had any profound impact on history.

Emperor Reign
Maurice Maurice.jpg 582-602
Phokas Phocas.jpg 602-610
Herakleios Herakleios.jpg 610-641
Constantine III* ConstantineIII.jpg 641
Constans II Constans2.gif 641-?
All unnamed Mid 7th century through 1300
Nikephoros III Nophoto.jpg Ascended before 1305,
incumbent at novel's end, 1320

(*)These are not named, but do not seem to have been affected by the POD.

Other Emperors

Justinian I is the emperor in "The Fake Pandemic", Harry Turtledove's sequel to Lest Darkness Fall by L. Sprague de Camp.

Leo III plays a posthumous background role in "Islands in the Sea", where he was the last emperor and was killed during the fall of Constantinople to the Caliphate in 718.

In the supernatural secret history, "Suffer a Sorceress", Alexios I is on his deathbed while members of his family employ magic in an attempt prevent his successor John II from takeing the throne. Alexios I is the POV character in "Two Thieves," where he is resurrected on Philip José Farmer's Riverworld.

Constantine XI is a POV character in "The Emperor's Return," which begins in OTL before segueing to a hypothetical future.

Other stories contain notable references to Emperors who died before the relevant Point of Divergence, or are set in OTL.