The Rival: The Third Book of the Fey (Fey, No 3)
Blue Isle, scene of a violent clash between its human native and the magical Fey, has now become the center of attention of the Fey king, Rugad. He is returning to finish the Fey invasion begun years ago but thwarted by the discovery that Blue Islander holy water was fatal to the aggressors. That stalemate has produced a wary coexistence, even intermarriage between royal families of the two races, but Rugad wants only one thing: total domination--and he will stop at nothing to get it