The Elixir of Death (Crowner John Mysteries)

the long version

County Coroner Sir John de Wolfe investigates a series of brutal murders in 12th-century England. The discovery of the murdered crew from a shipwreck ... is just the first in a string of atrocities. The captain was the husband of Sir John's former mistress, Hilda, and his investigation troubles both his shrewish wife Matilda and his current mistress. Then there are a series of horrific murders, all related by the use of an unusual knife and crossbows with Arabic lettering.