An Old Friend of the Family — Fred Saberhagen

Kate Southerland has been murdered. But she is not dead. Johnny Southerland has vanished, a severed, bloody finger the only clue.

The Southerlands are trapped in a blood-feud older than history. Their enemy is no mortal madman but the undying mistress of evil enchantment, Morgan Le Fay.

But the Southerlands are not Morgan's true target. She seeks to do battle with their protector, their defender, the only being who can save these mortals.