The Corruption is a malignant evil entity spoken of only in hushed voices, an ocean of darkness that flows from the top of Mount Morta. The Goddess Rea Dana tasked The Bergsons with fighting the Corruption. The corruption seems to corrupt animals into demons, and command various monsters to fight for it.
It seems that once, when Grandma Margaret was younger, the Corruption struck, but was beaten back somehow. Margaret has nightmares about it happening again.