It begins and ends with the Deep Black, the Great Nothing and Everything. The Abyss. It is the Paradox, both the source of matter as well as the end of matter. The Abyss is infinite, yet is nothing, and the Gods do not dare to tread there.
Still, the Abyss contains all life. Within its dark embrace are multitudes, perhaps countless numbers, of universes; each universe having its own unique composition. Within one of these universes, there resides an inconsequential speck of dust. This fleck of dust is the world of the Dead Wastes, a decayed and broken world. In the dominant language of this world, the planet has been named Valtuus.
In the universe of Valtuus, there are several alternate realities that lay atop each other like the layers of a rotting onion. These realities are all permutations of a common reality, known as Prime. From the innermost layer, or dimension, to the outermost layer you have the Infernal Dominions, the Wyld, Prime, the Gloom, and the Celestial Domains.
The various dimensions are mutations of the Prime, and in the case of the two innermost dimensions, the Infernal Dominions and the Wyld are actually perverse reflections of the Prime. If a right-handed swordsman were to enter the Wyld, he would find himself left-handed. Each dimension twists and distorts the features and properties of the Prime in a fashion unique to that dimension.
The Infernal Dominions are home to devils, demons, and daemons. Luftdamadon, Prince of Tyranny, commands the Legions of the Damned through his Devil Lords from the Impenetrable Fortress of the Ninth Portal. He leads his armies against the demonic hordes that often rampage through his Dominion and against the Shining Host of Sidar. The demons continuously fight each other as well as the devils and any other daemons caught in their path.
The Wyld is presided over by the Seelie and Unseelie courts of the Fey. It is a kingdom illusion and lies and the casual traveler should be wary of taking anything for granted, including the seemingly unbreakable laws of physics. The Queen of Mirrors is the current ruler of the Unseelie Court and her fey hold dominance over the Seelie Court whose throne is currently vacant.
Prime is the dimension on which all others are based upon. It is often considered a commodity, and the denizens and territories of Prime on Valtuus are skirmished over by the powerful entities of the other dimensions. Luftdamadon seeks to swell his armies with the ranks of souls, Sidar claims to protect these souls, the Queen of Mirrors believes these souls will eventually destroy her kingdom, and Tenebrous seeks to fuse all the dimensions under his control and reshape them in his own image.
The Gloom came under the influence of Tenebrous long ago, after being driven from the Celestial Domains by Sidar. Within the Gloom is the Realm of the Dead, which has become corrupted by Tenebrous. Tenebrous is the Dark Lord and commands a great undead host. He sent Karthisis, the Blood Tyrant to Prime through the Gloom Vortex created during the Long Night.
The Celestial Domains are ruled by Sidar, the Sun God. The Celestial Domains are often referenced as a heavenly paradise, but this is a misnomer. While many paradises exist in the Domains, not all realms of the Domains are welcoming, and many less than pleasant gods and entities dwell here. However, Sidar and his Shining Host do a fine enough job of maintaining order that most gods with evil intentions are relegated to secret schemes.
There exists a substance that forms barriers between the dimensions. In the beginning, this substance prevented all but the tiniest or most clever entities from slipping through. This substance is Æther, and it has become weakened overtime by powerful magic, harrowing events, entropy, and by time itself. In certain areas, the Æther is worn exceptionally thin and can be easily crossed by those with the knowledge. Temporary Gates can be created to traverse the Æther, but there are few permanent ones.
Beyond these dimensions are the Elemental Planes. The Elemental Planes include the Key Elements of Air, Fire, Water, and Earth. Where these planes intersect each other are the Composite Planes of Dust (Air & Earth), Ice (Air & Water), Lightning (Air & Fire), Magma (Earth & Fire), Salt (Earth & Water), and Steam (Water, & Fire). Access to these planes is most easily obtained in the Wyld, but a significant portion of various Elemental and Composite Planes also seep through the other planes. Such as the Flaming Pits of the Infernal Realms and the Dust Fields of the Gloom.
Small pocket planes and dimensions exist, and many of them are unknown to even the most powerful entities of Valtuus. Indeed, even more powerful beings could be biding their time within some unknown dimension, waiting to unleash devastation or even peace upon the known dimensions. There are even whispers among the devils that beings older than the universe slumber within the Abyss, and when they awake, they will consume everything.
This is just a sampling of the Cosmology of the Dead Wastes. I actually developed most of this before the release of 4th Edition D&D; originally being inspired by Wraith: the Oblivion and its Shadowlands, the Shroud, and Oblivion. I was mildly surprised when I read the new 4e Cosmology, and how similar the developments were to what I had laid down. The Elemental Planes were obviously inspired by [ripped from] Planescape and previous edition D&D Manuals of the Planes.
In the future I plan on detailing the inner and outer dimensions and perhaps some pocket planes.