Shadowsinger - L. E. Modesitt

Secca has succeeded in delaying the conquest of DeFalk by taking the battle to the enemy and sinking an invasion fleet. Yet even greater dangers loom because she can't be everywhere at once. She is newly married to her ally, Alcaren, the man who saved her life. Now, she must help him reach a higher level of sorcery than ever before. Richina, her young apprentice, must also redouble her efforts. Fearing a never-ending swarm of attackers from the home islands of the Sturrinese, Secca makes a bold and challenging voyage to those islands and conjures a massive eruption that wipes out most of the power base and population of the islands. But she must still face the most powerful wizard in the world, at the head of a great army and supported by many more wizards than she has ever had to fight before. As uncertain as ever of the extent of her abilities and of her stamina for endless battles, she desperately fights on against seemingly overwhelming odds.