Broken Blade, Book 3, the Rune Blades of Celi — Ann Marston

Book 3 in the Rune Blades of Celi series.
A kidnapped princess and a clanless man share a terrible secret-the dark powers of Maedun are preparing a new assault on the shinning Isle of Celi.
Using stolen Celae magic, the sorcerer Hakkar plans to rip down the veil of enchantment drawn about the island kingdom. Only the great sword Kingmaker can save the land. Until it, too, is shattered.
Brynda, daughter of Keylan, must teach her Rune Blade to sing death's song-or Maedun's Somber Riders will claim all Celi for their own!
...a lively mix of action, romance and cultural details...grabbed me from the start. - Locus