Sorcerer's Moon - Julian May

The stunning conclusion to a powerful epic fantasy from the worldwide bestselling author of the Saga of the Pliocene Exile.
For sixteen years King Conrig Ironcrown has ruled High Blenholme, battling both to preserve the Sovereignty he ruthlessly established over the four provinces of the island kingdom and to repel the invading Salka monsters that threaten them all. His hope for the future is his heir, Prince Orrion, whose betrothal to a princess of the province of Didion should assure the future peace of High Blenholme. But Orrion has no interest in the girl, and is determined to marry instead his childhood sweetheart, Lady Nyla.
Orrion's madcap twin, Corodon, dreams up a scheme to keep Orry and Nyla together by asking the supernatural Beaconfolk, who appear as lights in the sky, for a magical intercession. The twins are unaware that the Beaconfolk are fighting their own battle with others of their kind; to them all humans, even princes, are but pawns to be...