But not the book 
I am sure that I have read about a violent, bloodthirsty bishop, possibly related to William of Normandy, who travelled with the invading army, nominally to be responsible for spiritual matters, actually because he wanted to fight. But because he was a priest he was not allowed to draw blood with a sword, so instead he wielded a mace to crush people's heads.
Did this character exist, or an I conflating different bits of history, or even fiction with fact?