The Graystone Saga by Laura Klotz

One night in a rustic inn, a teenaged boy named Tobiah inadvertently gets involved in a fight revolving around a mysterious hooded woman. She turns out to be the fabled Lady Gray, a figure of legend that the majority of people fear. He ends up joining Lady Gray and her companion, the surly warrior Sabastian, on their quest to defeat and capture twelve demons, known as shades, who have escaped into the world. Only she, the bearer of the Graystone, has a chance of defeating the shades and returning them to their place of imprisonment. Gradually, Tobiah begins to piece together the truth about the lady, whose history is as tragic as her fate.