Lord Richard, Vampire Series by Nigel Bennett, P.N. Elrod

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.

Once called Lancelot, King Arthur's greatest knight, Lord Richard is now a vampire--but still sworn to protect the weak no matter the cost to himself. When a plea for help calls him to the rescue of a woman he had loved and lost, Richard must reconcile his ancient past and a lethal struggle to survive in the present if he is to ever find eternal peace.

An old enemy from Richard's past, Charon the Assassin, has returned and he is hell-bent on revenge. Only Richard can stop him, although theirs will be a battle that spans dimensions and centuries of time.