Wreckers Gate Book One of the Devastation Wars

A small band of soldiers is ambushed by raiders and forced to flee into the towering sand dunes of the Gur al Krin desert. The raiders let them go, knowing that none survive the desert and its firestorms. The small band seeks shelter in a cave, but their shelter turns deadly when a monstrous creature, fueled by an ago-old hatred of mankind, bursts out of its depths, slaying all but two. Drawn by whispers of power, the survivors travel deep underground, where they find a vast, buried wall. An ancient relic is set in the wall and they pull it free -- the wall sealed a prison built eons ago and the relic was all that held it together. Now a mad god is free and there is no one to stop him, since the gods who built his prison fled long ago.