Tartarus
The Planet is Hollow, divided into 9 different Layers. The surface has been abandoned due to various reasons, most long forgotten. Different kingdoms established themselves in each of the nine layers, which are all vastly different to each other. Each layer differes in flora and fauna, but also in such things as physics and what is assumned to be possible. The lowest of the layers is the one where the church lies, one of the strongest factions in the World. This layer is called Tartarus.