The Younger Gods (The Dreamers #4)

The four elder gods Dahlaine, Zelana, Aracia, and Veltan have ruled over Dhrall for eons. Every 25,000 years, the siblings pass on their duties to a quartet of young gods so that they can rest. But as the next changing of the gods approaches, the elder gods are faced with a potential catastrophe: an enemy has arisen from the vast wasteland in the center of Dhrall and is bent on conquering the entire realm and using all its inhabitants as nourishment for its minions. The Vlagh, as it is called, is a wellspring of evil, continually birthing nightmarish insectoid monstrosities to make up her army. But as the final battle looms closer, one of the elder gods begins losing her sanity. As the gods desperately search for ways to stop the Vlagh -- and rein in their unstable sibling -- heroes turn up in the unlikeliest places.