Carnage! A necromancer immigrated to the fort early on, which is not a problem. Everyone liked her, so she got to be mayor. Still no problem. She was mostly insane, which was more of a problem. Stumbling and babbling, typical DF insanity.
Then a giant bat lost its training and reverted to wild. Here's where I'm fuzzy on the chronology. I think the necro raised the bat who started attacking and killing other livestock which also got raised ...
Medievalists and Medieval-adjacent. Sort-of.