Lair of Bones Runelords book 4

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 sallies forth from the underworld, intending to put an end to mankind. In one last-ditch effort to heal the earth, the wizardborn Averan leads the Earth King, Gaborn Val Orden, far below the surface to the Lair of Bones, to face the leader of the reaver hordes.
There Gaborn must confront an ancient evil - before the world is torn apart.