A kingdom discovers a miracle substance which cures all ailments. Increasing their lifespan and removing the daily worries of death and disease propels the insular nation into a golden age of scientific, artistic, and magical wonders. This kingdom grows from a backwards peonage to a a pre-industrial juggernaut in a mere century, far surpassing the technological and magical prowess of their neighbors. Soon they set their sights on conquest and begin carving away at their massive neighbor.
As their discoveries replace the roles that their gods had in their lives, the gods begin to turn away from them. Soon, the gods stop answering their prayers and then a curse sets in, transforming them into horrible monsters. Their cities and the lands they conquered are now wastelands inhabited only by these "scourge." The king has hired adventurers to cleanse his homeland. Lured by the promise or riches, your mercenary troupe has answered the call.