The Navigator's Children - Tad Williams

The Hayholt is besieged by the Norns. Once the home of their immortal brethren, the Sithi, now capital of the kingdom of men, the fabled castle is under attack. And as the world is distracted by this strike against humankind, the Norns’ deathless witch-queen Utuk’ku turns towards the mysterious fateful valley called Tanakirú – the Vale of Mists. Meanwhile, Queen Miriamele hurries to save the Hayholt and capture the treacherous noble Pasevalles, but arrives to discover the traitor has escaped. And inside Tanakirú, Vale of Mists, the bond between Prince Morgan and Nezeru, a renegade Norn, has become something deeper and stranger than either of them could have anticipated. They journey ever deeper to the heart of the valley’s mystery, encountering wonder and horror, and come face to face at last with the ancient secret that has kindled the Norn Queen’s war – a secret that will destroy immortals and humans alike.