Planescape

As one know the D&D worlds are all connected via the planes. Heroes who have challenged every mortal deed have a chance to go to the place where immortals dwell, they can take the fight to the infernal realms, and journey to the lands of the dead. Although Planescape is designed with an Epic feel in mind, it also allows basic campaigns to be done in a world of myth... where the very gods are real. You can either play a mortal or planar being.    

The basic cosmology of the D&D universe is thus.... each campaign setting has its own world. These worlds are all connected to each other via Wildspace (Spelljammer). Yet the very nature of the universe, the prime material plane, is made up of the basic building blocks of creation from the Inner Planes... the elemental planes of air, earth, fire, and water. The inner planes also polarized by negative and positive energies, that in turn give rise to quasi-planes of each element, and as the main elements themselves border each other this gives birth to para-elemental planes. Lost among the Inner Planes are those of Metal, Wood, Blood, Bone, Flesh, and Spirit. On some worlds, these Inner Planes exist.

Surrounding the Prime Material plane and connecting to both the Inner and Outer Planes are the Transitives Planes .... Ethereal, Astral, Shadow, and Time. The Ethereal serves as a gateway toward the Inner Planes, and the Astral to the Outer Planes. The Plane of Shadows and Temporal Prime seem to be connected to all such planes. What makes things more interesting, is that the Ethereal Plane is a plane unto itself. The Deep Ethereal holds many Demi-planes which touch other planes. Demi-planes are peculiar unto themselves for they can connect anywhere and have there own set of rules.

The Outer Planes is usually the finally resting place for the spirits of mortals who believe in a particular power, and such the outer planes are the home of many gods and pantheons.The outer planes can either be divided between the ethos of Good vs. Evil or Law vs. Chaos. There is a constant struggle between pantheons, gods, and some of the planar races. Demons and Devil are constantly at war with one another, while the Celestials which to safeguard reality from the Infernals of the lower planes. Also the race of Modrons seeks to bring order to reality, while their counterparts the Slaadi seek to spread chaos. Most scholars view the planes as a Great Wheel, and at the center of this is the Outlands. Starting from the plane of absolute law, these groups of planes is usually considered the Celestial/Upper Planes (Mechanus, Arcadia, Celestia, Bytopia, Elysium, Beastlands, Arborea, and Ysgard). Starting from the plane of absolute chaos, these groups of planes is usually considered the Infernal/Lower Planes (Limbo, Pandemonium, Abyss, Carceri, Hades, Gehenna, Baator, and Acheron).