Three hundred years ago, something terrifying arose and pushed humanity to the brink of extinction.
Now, a small remnant - descendants of the survivors who were able to escape the massacre below - lives above the clouds, on the top of a Mountain.
When they discover that their water supply is being poisoned Down Below, an expedition, including seventeen year-old girl Icelyn Brathius, must descend and face the monsters, the Threat Below, that wiped out civilization centuries .