When I was putting together my Call of Cthulhu game set in ancient Rome, I scoured as many of the mysteries by Steven Saylor or Lindsey Davis as I could find, looking for any idea that I could give a "Mythos twist" as well as a number of original source histories. I imagine the same would work with medieval mysteries, by, say, Michael Jecks or Margaret Fraser, or with the histories of real life figures that figure in those novels.
In one series of the Fraser novels, the heroine is a nun who wants to stay in her cloister, but these bodies keep popping up with a severe case of death. In another the hero is attached to a troupe of wandering players. In the Jecks series the heroes are a retired knight with a secret, and what is, essentially, a tax collector, but these bodies keep popping up with a severe case of death.