The Fey, book 3 : The Rival
Kristine Kathryn Rusch2013

The Fey have conquered half the world with their great skills — not just at combat and with powerful magicks. The Fey have another weapon, a greater weapon: Patience. Twenty years after their first invasion, the Fey try again to conquer Blue Isle. This time, the greatest tactician the Fey have ever known, the Black King Rugad, leads the fighting force. He’s after not only the strategic island, but he’s also after his own great-grandchildren. The oldest of those children has just come of age. He’s first in line to inherit Blue Isle’s throne. But as Rugad appears on Blue Isle, that Isle’s King Nicholas must move beyond guerrilla war to an out-and-out fight for dominance, using a magic created in the union of Fey and Islander, a magic that no one from Rugad to Nicholas himself could ever have expected…

Titre original : The Fey, book 3 : The Rival (2013)