The Fey Series by Kristine Kathryn Rusch

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 total domination--and he will stop at nothing to get it.

For a conquered people, a long-forgotten magic holds their only hope. Blue Isle's capital lies in ruins, leveled by the new invasion force of Fey. But to hold his conquest, Black King Rugad must capture his two half-Islander, half-Fey the fiery Arianna and the gentle Gift. For one of them will inherit the Black Throne, and Rugad will not rest until he owns them both, body and soul. Yet, in the wily and courageous Arianna, this legendary warrior may have met his match... moreFor a conquered people, a long-forgotten magic holds their only hope. Blue Isle's capital lies in ruins, leveled by the new invasion force of Fey. But to hold his conquest, Black King Rugad must capture his two half-Islander, half-Fey the fiery Arianna and the gentle Gift. For one of them will inherit the Black Throne, and Rugad will not rest until he owns them both, body and soul. Yet, in the wily and courageous Arianna, this legendary warrior may have met his match. Fugitives in a wasted land, she and her father, Blue Isle's King Nicholas, plan the destruction of her ruthless great-grandfather. Drawn to a forgotten sacred place in the shimmering Cliffs of Blood, they will find Arianna's long-lost brother, Gift. But an old enmity stands between Arianna and the reluctant warrior Gift, and an ancient Fey prophecy darkens their future--and that of the world. The brilliant epic of the Fey continues in a blaze of beauty and savagery, treachery and enchantment. less