The Mage Storms Trilogy
Book 3: Storm Breaking

Duke Tremane's allegiance to his emperor is stretched to breaking point. When besieged citizens of Hardorn offer him the chance to sit on its throne, he can see no point in refusing. The antiquated earth-taking ceremony of knightship proves he has an unsuspected talent for earth magic. It is a talent that will bind him to the future not only of Hardorn, but to his former enemies in Valdemar. When the final storm breaks, none but the most valiant will survive.