Today is the anniversary of the fall of the Orthodox Roman Empire incorrectly labelled by the west as ''byzantium'', it fell because the emperor and most bishops except St Mark of Ephesus and his allies, betrayed the Orthodox faith, by signing the Union of Florence (1439) and capitulated to the Pope of Rome, it was later rejected of course.
The last emperor was Constantine XI
The Orthodox do not consider Constantine XI a saint, though there are some who incorrectly, personally consider him so because of their love of the Byzantine (Roman) Empire. However, the last Emperor has never been recognized as a Saint due to his heretical beliefs and his compromising of the Faith in order to gain material aid from Latin lands. When the Empire was in need, he sought Latin military aid by reaffirming the heretical statements of the Council of Florence.. And it is said that he defiled the Church of Agia Sophia the day/night before the Fall with a blended Divine Liturgy and Latin mass. In attempting to preserve an earthly Empire, he lost a Heavenly Kingdom. In contrast, there is Tsar Lazar of Serbia, who gave up an earthly Kingdom for the Heavenly. The Faith is always more important than earthly possessions.
Here is a brief info about St Tsar Lazar:
According to general and ancient folk tradition, an Angel of the Lord appeared to the saintly Prince Lazar before this battle and asked him which kingdom he wished to choose: would he choose the earthly kingdom or the Heavenly Kingdom. After prayerfully pondering the question, the Prince, who yearned for heaven, replied to the Angel of God: “If I should choose the earthly kingdom, it is only for a brief time, and is momentary and transitory; but the Heavenly Kingdom lasts always and forever.” Thus the God-loving Serbian ruler decided in favor of the Kingdom of Christ the king in the heavens.
By St. Justin Popovich, Life of St. Lazar quoted from:
(https://followthesaints.com/books/the-heresy-of-patriarch-kirill/the-heavenly-kingdom-st-lazars-choice)