The planes of existence may be thought of both as places and as states of being. They exist in relationship to one another in the three usual spatial dimensions, as well as in higher dimensions that are difficult for mortal beings to perceive and understand.
Material plane. The material plane is the mortal realm—the normal physical world. It is sometimes known as the "Prime Material Plane," to distinguish it from any other material planes that may exist.
Elemental planes. The elemental planes are four great expanses of raw elemental matter: the planes of air, earth, fire, and water. They are home to elementals (beings of intelligent fire, water, and so on), as well as to more unusual creatures capable of living in such extreme environments.
The mortal world is thought to exist at a point where three of these four planes converge: the plane of air forming the sky, the plane of earth forming the ground, and the plane of water forming the sea. It is believed that the plane of fire also lies somewhere nearby—close enough to warm the world while far enough not to incinerate it.
Ethereal plane. The ethereal plane is a misty world that exists in parallel to the material plane. It is known by many names, such as the Spirit World, the Shadow World, the Dream World, and the Land of Ghosts.
A creature on the ethereal plane is said to be ethereal. Some creatures, such as ghosts, are naturally ethereal. Other creatures, such as phase spiders, can shift between the material and ethereal planes at will. An ethereal creature can move in any direction as easily as walking, and can pass right through material objects. The material world is visible from the ethereal plane, but it appears grey, foggy, and indistinct. Beyond a distance of 60 feet, only vague shapes of light and darkness are visible. Ethereal creatures are normally invisible to creatures on the material plane.
The ethereal world and the material world do not normally interact, but some magical effects can pass between them. Most notably, magical force effects (such as Murphy's magic missile) extend across both the material and ethereal planes.
Faerie. The fairy realm lies somewhere on the other side of the ethereal plane. Here, the lords and ladies of the fey have their strange courts. Many fairies and fairy-creatures can travel between fairyland and the mortal world, passing along hidden paths through the ethereal plane (though some fey are so closely connected to the natural world that they dwell only on the material plane).
Long ago, the high elves came from Faerie to make their homes in the mortal world. To this day, elves who have seen too much sorrow, who in the course of their long lives have grown weary of all the world’s pleasures—these elves begin to hear a distant voice calling them, until at last they seek out the hidden ways that lead them out of the mortal sphere forever and back to the realm of Faerie. Whether they go there to die, or to live forever, or for some other purpose—is a secret they do not share.
Astral plane. The astral plane is the plane of stardust, an endless expanse of inky darkness and twinkling light. It exists within and between and around the other planes.
On the astral plane, the normal laws of space and time do not apply, and it is possible to travel great distances in a brief moment. Teleportation spells work by transporting their subject through the astral plane. A short teleport requires only the slightest skip across the boundary of the astral plane, which is relatively safe. A longer jump requires diving deeper into the astral plane, and is correspondingly more dangerous.
Upper planes. The upper planes are the home of the gods, along with angelic beings and the ascended spirits of righteous and enlightened mortals. The upper planes are also known as the tiered heavens, for they exist in a ladder of multiple levels. There are at least nine heavenly planes, each higher and holier than the one below it. Beyond these there may be others that are higher and holier still.
The nature of the upper planes is shrouded in mystery. Wizards who have attempted to visit them report that they are suffused with an intense energy that is deadly to living things. The Temple teaches that this is the holy presence of the gods, which sinful mortals cannot withstand. Some mystics say that the upper planes represent a form of existence so profoundly real it causes mere physical matter to wither away.
Some have managed to project their spirits to the upper planes and communicate with the wise beings who dwell there, but many more have been driven mad in the attempt.
Lower planes. The lower planes encompass the shadowy realm of the death god Gnagk, the nine circles of Hell, and the endless demonic kingdoms of Pandemonium.
Please note that in this game a sharp distinction is not drawn between demons and devils. The term "demon" suggests a wrathful and destructive evil being, while "devil" suggests a creature of a more cunning and deceptive disposition, but both demons and devils are inhabitants of the lower planes, and they share broadly similar goals—namely, to feast upon the souls of mortals.
The Abyss. The Abyss is a cosmic dumping ground: the great waste basket at the bottom of the universe. It is a plane of utter chaos, filled with everything so horrible it was cast out of the world by the gods.
Anything that "falls out" of the other planes eventually lands in the Abyss. In ancient times, spells and weapons were created that could rip their victims loose from the planes and hurl them into the Abyss. These forbidden arts were lost long ago—hopefully for good.