A Russian author quoted by Kissinger in Diplomacy claims that Russia is the continuation of The Roman Empire, not sure this is just nationalism or a grounded thesis.
Going with The Meme[0], Constantine XI[1] was the last Byzantine emperor until his death and the Fall of Constantinople in 1453. The line of succession, ignoring dead empires and bought titles, is supposedly brother (Thomas) > nephew (Andreas, son of Thomas) > niece (Zoe, sister of Andreas) > Grand Prince of Moscow (Ivan III, through marriage with Zoe) > Tsar of the Russian Empire (grandson of Ivan III)
[0] https://i.imgur.com/tolQ9ih.png [1] https://en.wikipedia.org/wiki/Constantine_XI_Palaiologos