The Evil Below
The bit of reality popularly deemed Aithir, which contains the Dragon Empire, Thorizdunthe Dwarf-King's Under-Mountain, the Hierophant's untamed forests, is hardly the whole of reality. Indeed, Aithir is but one World among many. Scholars theorize that there are potentially an infinite number of discrete Worlds in addition to the one in which we live.
The High Arcanist Seroth calls Aithir the Known World, and this designation has been formalized throughout the past few centuries as the practice of magic became increasingly reliant upon Seroth's organization to concentrate knowledge and streamline training in the arcane arts.
Three other Worlds intersect with Aithir: Sidhe and The Fade overlap parts of Aithir (albeit not contiguous parts), and The City of Countless Doorways can be accessed by arcanists with sufficient skill.
Cosmologists insist that Infernus, the home of demons and devils, is not a World in the strictest sense of the term: rather than being a complete World with discernible properties and natural/magical laws, Infernus consists of multiple lands that remained unfinished or otherwise incomplete. What we call Infernus actually consists of the bits of reality (lands, ideas, even paradoxes) abutting Aithir that could not be included in reality proper or that were, strictly speaking, impossible to exist given Aithir's laws of magic/physics.