David Farland - The Lair of Bones (The Runelords #4) (2002)

In THE LAIR OF BONES the stars fall from heaven and the very earth trembles in pain. With Gaborn's kingdom of Mystarria in ruins, four powerful kings march to claim its spoils, even as a vast army of reavers from the underworld sallies forth, intending to put an end to mankind. In one last-ditch effort to heal the earth, Averan leads the Earth King, Gaborn Val Orden, far below the surface to the Lair of Bones, to confront the leader of the reaver hordes. There Gaborn must confront an ancient evil - before the world is torn apart. In this fourth volume of THE RUNELORDS series, David Farland continues to rewrite the boundaries of epic fantasy. There are only impossible decisions for his protagonists, and the evil of the reavers may simply be a mask for the true horrors to come.