Oblivion's Forge: Book One of the Aona by Simon Williams

For thousands of years they have sought the world from which they were cast out. Now, at last, Aona has been found. The younger races of this world will all be swept up in a struggle for survival, as their ancient, malevolent masters, guardians of all Aona's secrets, rise to do battle with their foes, remorseless destroyers of world after world throughout the known Existence.