Heretic The Grail Quest Series, Book 3.

Thomas of Hookton is a seasoned veteran of King Edward's army and a natural leader in what will be remembered as the Hundred Years' War. Accompanied by a small but able band of soldiers, Thomas is sent to Gascony to capture the castle of Astarac. But he has ulterior motives for accepting the charge: Gascony is the home of the black knight, Guy de Vexille, who brutally slaughtered his father. It is also reputed to be the place where the Grail was...