Tanith Lee 
Castle of Dark 
The Castle of Dark Book 1

Although she leads an overprotected life with the two old hags, Lilune knows she possesses a special gift. When she 'calls’ the musician, Lir, to her prison-like castle, she knows she must avail herself of the opportunity to escape and explore the world. But travelling south of the castle, Lilune and Lir realize that they aren’t alone – for an ancient, infectious evil accompanies them, which instills terror in everyone they meet. Lir dislikes arrogant Lilune, but finds himself intrigued by her and the source of the evil. Is it within Lilune, or does it come from a deeper source? When the pair become separated, he carries on searching for her. Finally, Lilune returns to her castle in despair, believing that she must be imprisoned to protect the world from the evil within her. But Lir follows her, and discovers that the root of the evil lies deep beneath the castle…