The Death Whisperer. The Burning Tree, #1

When young Greesha whispers a single word to her wicked uncle as he sleeps, she discovers that she has a terrifying gift. She has the power to destroy vampires with nothing more than her voice. Speaking with her dead grandmother in an apparition, Greesha learns that she is a 'Death Whisperer'. One of her kind is born only every nineteen years, during an extremely rare lunar cycle. Greesha's grandmother tells her that the forces of evil will reach out to find her and destroy her and tells her that, in order protect herself, she must - in turn - reach out with her mind to find and unite with those who are good and righteous.
As vampire forces gather and ancient prophecies unfold, Greesha discovers her destiny is intertwined not only with legendary figures of her own time, like the Pictish queen 'Kiffan the Defiant', a Vampire Slayer who blunted the Viking invasions of Scotland, but with her descendant, Annis, Queen of the Highlands, far in the future.