Conqueror's Moon: v.1 by Julian May
A major new epic fantasy from the worldwide bestselling author Julian May. High Blenholme is a lush but treacherous island comprised of four feudal domains, three of them owing fealty to the fourth, Cathra. The domains are in perpetual political turmoil and border skirmishes are commonplace. The island is not only under constant attack from marauding sea-monsters, inhabitants of nearby islands, but also from the demonic woodland outlaws called Green Men, and overhead looms a giant moon, possible source of otherworldly beings who have been known to grant magical gifts. When Bram, the Sovereign King's younger son (also an apprentice alchemist), attempts to help his elder brother escape an arranged marriage by invoking these alien creatures, he unwittingly sets in motion a chain of events that will lead to cataclysmic civil war.