Heretic is #3 in Bernard Cornwell's Grail Quest series set in the 14th Century. Thomas Hookton is an English archer whose father was believed to possess the Holy Grail. Thomas's father was killed by those who wanted the power they thought the Grail would give them. In this story Thomas goes to Gascony in France to learn about his family and seek out his father's murderer. Many adventures and battles follow.