Prince of Darkness — Sharon Kay Penman

The Edgar®-nominated author of the medieval mysteries featuring Justin de Quincy places the Queen's Man far from home -- and in the presence of a most cunning foe...

Justin de Quincy has been lured to Paris by his nemesis, Prince John, on a mission of mercy. The prince is suspected in a plot to kill his brother, King Richard. Despite John's hunger for the crown, he's unwilling to put himself at risk for regicide -- and he wants Justin's help in discrediting the document that implicates him.
 
Justin only concedes to John's request when he realizes that the welfare of the woman he serves, Eleanor of Aquitaine, is also at risk. It is a concession that will take him to a bloody chamber at Mont St Michel, to a putrid dungeon in Brittany, to a murderous encounter in a Paris cemetery, and to the unraveling of a conspiracy that might have changed the course of English history.