The Fey. Sacrifice - Kristine Kathryn Rusch

The Blue Isle has been free to rule itself for eons - separated by the oceans from a mainland under siege. But now the Fey have finally broken through the Isle's best defences, natural and supernatural, and the islanders must learn to live with the exotic and warlike Fey. For reasons they do not understand, the Fey then begin to lose ground falling prey to a vicious poison that annihilates them on impact. They retreat to the Shadowlands - a fortress hidden deep within the forest, in a magical other dimension. Jewel, granddaughter of the notorious Black King of the Fey, can see a way out of their dilemma but it will mean making the biggest sacrifice a Fey has ever been required to make. and it will make an enemy of her otherwise loving father.