Conqueror's Moon — Julian May

From the author of The Many-Colored Land comes the saga of a land beyond the horizon, where the quest for power is eternal, where magic and mystery are feared above all, and one man sought to reign.
 
On a remote island, far in the Boreal Sea, four kingdoms have struggled against one another since time out of mind. Most mysterious is the marshland kingdom of Moss -- feared by the others and ruled by the Sorcerers. Soon, all will be put in peril's way.
 
In recent years, three of the kingdoms have suffered fearsome volcanic eruptions that choke crops, famine among people, and an ailing leader on his deathbed. Only Moss, poverty-stricken and desolate at the best of times, seems untroubled.
 
But Prince Conrig of Cathra, who waits patiently as his father, the king, wastes slowly away, is in league with his lover, the seductive sorceress Princess Ullanoth of Moss. And if their secret alliance succeeds in its goal, the warring kingdoms of High Blenholme will be united once and for all -- under the iron hand of one supreme rule.