The Threat Below (A Brathius History) (Volume 1)

Three hundred years ago, something terrifying arose and pushed humanity to the brink of extinction. Now, a small remnant – the descendants of the survivors who escaped 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 Icelyn Brathius, must descend and face the monsters, the Threat Below, that wiped out civilization centuries ago.
Icelyn quickly learns that all is not what it seems as she uncovers secrets hundreds of years old and struggles to stay alive in a world where no human is fit to survive.