Every twenty years, one specially trained and gifted Keeper journeys into the dark cavern from whence the demonic horde arrives. Far beneath the daylight, the Keeper uses magic to re-energize the iron seals holding the demons from their human prey. But this year, something is wrong. 
Demons spill from the cave entrance before they should be able to pass out of their prison.  They wreck havoc on the Keepers, killing the woman training to entrap them and slaying the king.