What is "The Opustoshen" in the novel, Earthborn?

The Opustoshen mentioned in the story is a large desert-like region outside of the Gornaya thought to be harmful to all forms of life whether Earth, Sky or Middle peoples. During the course of the story, it is discovered that the Opustoshen contains the remains of many thousands of human bones, presumably the scene of a large battle, and is also where the Rasulum Leaves are found.