The Gate of Gods [Fall of Ile-Rien 03] - Martha Wells

Tremaine Valiarde and a small, brave band of heroes ventured into a wondrous new realm on their desperate mission to save Ile-Rien from the conquering Gardier. Now, as a relentless enemy creates chaos and destruction - with the fate of the magical city of Lodun hanging in the balance - the last hope of a land besieged may rest on the far side of a secret portal.