Who inhabits Netherworld in the novel, Wolfsbane?

The Netherworld is a place that is like the anti-Earth, a place where strange creatures live. Magicians and witches over the years have learned how to call the creatures of the Netherworld to Earth and one witch made a pact with the leader of the Netherworld to take over Earth and rule the two realms together. This is the origin of the Witches' War or the war between the Keepers and the Searchers.