Fire Sea (Death Gate Cycle #3) by Margaret Weis, Tracy Hickman

Abarrach, the Realm of stone. Here, on a barren  world of underground caverns built around a core of  molten lava, the lesser races -- humans, elves,  and dwarves -- seem to have all died off. Here, too,  what may well be the last remnants of the once  powerful Sartan still struggle to survive. For Haplo  and Alfred -- enemies by heritage, traveling  companions by necessity -- Abarrach may reveal more than  either dares to discover about the history of  Sartan... and the future of all their descendants.