Beneath Ceaseless Skies - My Sister's Soul by Erin A. Tidwell
She looked dead, as well she would be if she were anything else. Only a powerful magus could live without a soul. But even Khatereh needed the touch of her soul once in awhile; on midsummer day, as was her personal tradition, she would let it rest within her and be renewed. With her soul inside her body, she would be as vibrant and beautiful as a twenty-two year-old sister ought to be, and when it was removed she would look healthy again, for a while.