Conqueror's Moon Julian May

The four kingdoms in the Boreal Sea that have clashed since time out of mind. But Prince Conrig of Cathra--who waits patiently as his father, the king, slowly dies--is in league with his lover, the seductive sorceress Princess Ullanoth of Moss. And if their secret alliance succeeds, the warring kingdoms will be united once and for all--under the iron hand of one supreme rule.