The Netherworld is one of the planes that comprise the Tetraverse. Like the Astral Plane, the Netherworld is a plane of the mind - notably, emotions and ideas. However, these concepts are often twisted into negativity, making the plane and its inhabitants naturally evil.
The Netherworld was created at the same time as the Astral Plane through the efforts of the Ten Voidgods. Its purpose was to add an element of dualism to the Tetraverse, in an effort to provide challenges and disruption to the creatures of the Material Plane. These challenges, along with the possibility of enlightenment and eternal life within the Astral Plane, provides the spiritual focus and conflict for the beings in the Material Plane.
The Netherworld's appearance is the result of various feelings and ideas present within its beings' minds. These feelings are often negative ones, such as hatred, despair, greed, and the like. Due to this, the plane has a twisted and horrific appearance, and beings who are not native to the plane often endure extreme pain and suffering within it.
Whereas most of the plane has this sort of evil atmosphere, the lairs of the plane's ruling lords (Called Archdemons) are ornate and fancy, though still ominous. These lairs are the only locations within the plane that do not bring suffering upon its denizens; in fact, the Archdemons and their followers often relish in food and vile festivities.
Much of the Netherworld contains basic physical laws like ones that can be observed in the Material Plane. However, since much of the plane's state is based on the minds of the Archdemons, there are some physical features that can be considered "unnatural" to dwellers of the Material Plane. Pieces of land can float within the air, and buildings can sail through the sky like ships. There are also many odd land formations that would be impossible to see on a planet in the Material Plane.
The plane is ruled by the Demon-King Asmodeus. The Archdemons serve him directly, and in turn have their own servants and followers. The majority of the plane consists of demonic beings that are innately evil. All of these beings are native to the Netherworld and have an extremely strong affinity with the purest forms of magic, to the point where they are able to construct war machines and other weapons by harnessing its infinite power.
The Netherworld also consists of many non-natives. These are usually former denizens of the Material Plane whose minds have been corrupted by sin (See: Descension). These beings have no connection with the Netherworld and thus do not have the power to shape it to their liking. Because of this, they are often tortured by the natives for pleasure and amusement. Unfortunately for the non-natives, the mind is the only thing that matters within the plane, not the body. So, whereas their bodies usually die off fairly quickly upon arrival, their conscious minds are made to endure an eternity of torment.