Keeper of the King by Nigel Bennett
Forces greater than even the magic of Merlin granted him his dark immortality. Over a thousand years ago he was Richard d'Orleans, undefeated champion of Normandy, then he was Lancelot du Lac, Arthur's greatest knight. Now he is Richard Dun, security specialist in modern Toronto, but still he hears the irresistible summons of the old ways and...answers them. To save the woman he loves above all else, he must do the impossible. Once more, the last defender of Camelot must begin anew his quest for the grail...and this time to succeed - or die forever.