Autochthonia is the world-body of the Primordial Autochthon, who fled Creation after the Primordial War with a small population of humans. Autochthonia is now separate from Creation, home to the Eight Nations and the Alchemical Exalted.
Also called "The Realm of Brass and Shadow," Autochthonia is a sunless wilderness of metal and machinery through which entire nations drift on unpredictable courses. Tubes, pumps and ducts coursing through the realm deliver water, nutrients and other raw materials to human communities, and in return the humans labor to maintain as much of world as possible to keep Autochthon alive.
The Eight Nations of Autochthonia, clustered around its Pole of Metal, have developed similar highly rigid social structures in order to manage scarce resources, and any deviation from orthodoxy is likely to get one branded a Voidbringer and executed or exiled. They engage one another in war, trade or diplomacy when they are near enough for safe travel. They also create Alchemical Exalts to defend and serve state interests.
Beyond the Eight Nations are the numerous tunnels and conduits of the Reaches, inhabited by exiles from civilization, Voidbringer colonies, horrific mutants, and mechanical spirits. These areas are also full of "natural" hazards such as toxic fumes, exposed machinery, arcing electricity and steep drop-offs. They are also riddled with Blight Zones, manifestations of Autochthon's illness, from which gremlins strike out at humankind.
Autochthonia is cut off from Creation by the Seal of Eight Divinities. This barrier is impenetrable from the Creation side, but could be opened from Autochthonia's end by sufficiently powerful magic. Autochthonia also has a potential motivation to engage with Creation: after over four thousand years, Autochthon is dying, the incidence of Blight Zones is increasing and the population is approaching a ceiling determined by the number of available human souls.