Khyber

Khyber lies beneath Eberron, and is said to be the body of the progenitor dragon that it is named after. It is a massive subterranean realm, similar to the Underdark of other settings. The topmost layers of Khyber are pretty much what you would expect from being underground, layers of stone and soil. The layer is littered with natural caverns, and subterranean ruins from the Dhakaani empire or ancient dwarves.
As you go deeper, Khyber becomes more and more filled with demiplanes. These pockets of space operate on their own rules and logic, and the rules that govern the Material Plane are unreliable if they apply at all. It is these demiplanes that are the source of most of the "unnatural" things in Khyber. The fiends, and the Daelkyr, are imprisoned in their own demiplanes within Khyber.
Functionally, the demiplanes of Khyber behave similarly to the layers found in the other planes of existence. Each demiplane has a finite amount of space within it, though they can vary wildly in size. The edges of a demiplane are inconsistent, they could be bordered by a wall of natural stone, walls of force, or they could just loop around. Travel to and from these demiplanes requires the use of portals, and these portals are fairly common within the tunnels and caverns of Khyber. Some of them are imperceptible, a tunnel seemingly leading directly into a demiplane. Many of these portals have been sealed, either by the dragons of Argonessen or the Gatekeepers, preventing travel both into and out of the demiplane. This is the case for the prison demiplanes of the Daelkyr.
Due to these demiplanes, Khyber is not bound to the physical space it occupies within Eberron, nor does it need to be spatially consistent. Portals to demiplanes can be anywhere within Khyber, and there can be multiple portals in vastly different places that lead to the same demiplane. One such example of this is the prison demiplane of Dyrrn, the Corruptor, which has portals beneath the Eldeen Reaches and also beneath the Mror Holds.
Whilst some would argue that Khyber is not itself a plane of existence like the thirteen planes, it behaves in ways that are consistent with these planes and it is useful to group it with them. The demiplanes of Khyber may defy the natural laws of Eberron, and time and space may behave in unnatural ways. Where Khyber differs from the planes, however, is in a lack of unifying theme. All of the layers of Fernia share a concept, Fire, but this is not the case in Khyber. Every demiplane is wholly unique. Another difference between the planes and Khyber, is that the demiplanes of Khyber never contain any of Eberron's celestial bodies. There might be moons, rings, or stars in the sky, but they won't be the Moons of Eberron, and the ring won't be the Ring of Siberys.

Type

Underground / Subterranean