The Craft of Light (Night-Threads, No 4)

Three unsuspecting visitors arrive in a world of magical adventure, only to discover an ancient Thread-wielder has summoned them. For this sorceress, shapeshifter, and soldier, each is destined to play a vital role in restoring peace to the throne of the duke, Aletto, the rightful heir of Zelharri. But secret powers are at work, and for Lialla, the outcast duchess, intrigue and peril finds her at every turn. And while her powers are fierce and fearsome, the magic known as Hell-Light has an uncanny way of determining your fate. What had seemed a time of peace has suddenly erupted into something far more dangerous than any could have foresaw.