The Skull of the World: Witches of Eileanan #5 - Kate Forsyth

In the land of Eileanan, the Pact of Peace has not meant the end of unrest. Isault and Lachlan must continue to deal with their military position in Tirsoilleir—where a spy has compromised their tactical secrets, leaving them at a tremendous loss.

Isabeau the witch longs to rejoin her sister and her home in these uncertain times, but she must first complete one last task for her teachers. To find her true purpose in life, she must reach the Skull of the World and listen to the silent words of the White Gods. Few survive. But those who do are changed forever.

Now, when she does return home, she does so with an extraordinary gift. But when Margrit of Arran decides to take advantage of Lachlan's absence by kidnapping his son and heir to the throne, Isabeau must learn to use her gifts in the face of evil—and overcome her enemy once and for all.