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