We know that Fyodor was around when Fukuzawa was 32

and he also appeared in chapter 2 of the anime, season four, and with the new chapter, we can say that he was alive and looked the same age DURING the medieval era

It's even said by sigma that he looks the same age (Chapter 111 for reference on how he looked in the prison arc)

And we can assume it was placed when Bram still had his duke position, and what is my point?

He is old, very old and his ability is to take the rest of the years a non-ability user has. It hasn't been fully said what Fyodor's ability is about, we only know that he can kill non-ability users with a touch, and it would make sense for his ability to take the rest of the years those people would have lived, the non-ability users, but why am I only saying non-ability users? because it hasn't been shown Fyodor killing any ability user with his ability, neither Nikolai nor Sigma died while holding his hand, so I believe it's safe to say that his ability only works with non-ability users; let's remember that the ability is tied to the user's soul, and going by this theory by taking the rest of the years of those, his ability somehow gets a fragment of the soul away, it would give more years on his life making his aging less faster and making him live more.

Also! the ten calamities are said to be able to destroy the world, and yet again it would make sense for fyodor to be one due to 1 his intelligence and his ability, if someone kills a large number of people, can the world be considered as not destroyed? it can and at the same time can't because there would still be people, however the population would decrease a LOT, and if fyodor teamed up with bram to destroy the world they perfectly could by fyodor killing off every non-ability user and by bram a vampire outbreak on the rest of ability users and make them kill eachother.

HOWEVER THATS JUST A THEORY

A BSD THEORY