Sorcerer's Moon

From the front flap of the first UK edition: "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 continued peace for 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 intersession. 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 used in their own conflict. Their granting Orrion's wish comes in a manner the twins far from expected, and precipitates chaotic fighting amongst the folk of High Blenhelme. As battles rage both on the ground and in the sky, the only hope for peace can be found deep in King Conrig's murky past. His former spy, Deveron Austrey, has secret magical powers and no love for the Beaconfolk. And while many of his subjects no longer remember the King's first wife, Maudrayne, she has never forgotten that her son is the true heir to the throne of High Blenhelme."