1 Part thought experiment, 1 part inspiration generation: Which historical figures make sense as undead? liche, vampire, otherworldly creature of death? I think we can count Rasputin and Vlad Tepes by default. Valerian makes sense, for Rome. Persian mysticism combined with arrogance and wanting a puppet ruler for their largest rival. They do some magic-handwavy shenanigans and boom, you have Vamperor Valerian. He has the traditional vamp powers with necromancy as an easy side twist. Zombie and skeleton Roman legions ensue.
Historical figures... as undead... ooooookay.... Ivan the Terrible: He was kind of insane to begin with, killed indiscriminately, and was prone to bouts of paranoia. Sounds like a vampire to me. Queen Victoria: When you rule for that long, there has to be a reason. This probably isn't it, but think of the changes it would cause to the entirety of European politics, as so many of her descendants were controlling other countries! Cesare Borgia (not to mention Rodrigo and possibly Lucretia): Do I need to explain this one? Frederick Barbarossa (HRE): Big, thugly man with a red beard who overpowered everyone around him? Could be a vampire. Ghengiz Khan: Vampire horsemen (Eastern Knights?) riding down the peasants and terrorizing the countryside? I'm sure I could think of more if I was more awake...