Cloudbearer's Shadow: Book 4, the Rune Blades of Celi

Gareth is the youngest and the last of the unfortunate lords of Skai. Called home from a lonely exile, he finds his father fallen into shadow. And worse. For the Maedun conquest of Gareth's homeland is complete. The standing stones are silent, their webs of magic torn asunder. The Rune Blade called Bane has been lost forever to dark sorcery, and the somber riders rule the islands that were once home to Gareth's people.