Planescape
Planescape is a fictional campaign setting for the Dungeons & Dragons; fantasy role-playing game. Planescape is unique among D&D settings in that it encompasses an entire cosmology, which itself encompasses all of the other Dungeons and Dragons worlds, linking them all via inter-dimensional magical portals. Planescape, as it name suggests, is comprised of numerous planes, or dimensions. The planes can be grouped into three basic categories: the spatial planes or substantial planes (containing solid matter, free-standing energy and living creatures); the transitive planes (connecting the other planes and generally containing little, if any, solid matter or native life); and the demiplanes (minor, artificial planes, often possessing qualities of both). The spatial planes include the Material Plane, the Inner Planes and the Outer Planes; the transitive planes include the Astral Plane, the Ethereal Plane and (according to some sources) the Shadow Plane.
The planes as a whole are usually presented as a series of concentric circles, with alternating spatial and transitive planes; from the center outwards, they are order as follows: Inner, Ethereal, Material, Astral, and Outer Planes. (The Shadow Plane, if it is included, is separate from the others, as is usually represented as being connected to the Material Plane.) Demiplanes, although most commonly connected to the Ethereal Plane, can be found attached to any plane. All planes, save the demiplanes, are infinite in scope and size.
The Outer Planes are physical manifestations of moral and ethical alignment, of belief, and of the will of the gods. Most of the Outer Planes are subdivided into layers, which are essentially sub-planes that represent one particular facet or theme of the plane. (For example, Baator's geography is reminiscent of Hell as depicted in Dante Alighieri's The Divine Comedy.) There are seventeen Outer Planes in total.
The Outer Planes are typically represented in a ring, with the Upper Planes (the planes of Good alignment) at the top and the Lower Planes (the Evil planes) at the bottom; the Lawful Planes are at the left and the Chaotic Planes the right. Clockwise from "top", they are:
Geometrically, Sigil is a torus; the city itself is located on the inside of the ring. There is no sky, simply an all-pervasive light; over the edge of the ring is an infinite, impenetrable void. Sigil cannot be entered or exited save via portals; although this makes it quite safe from any would-be invader, it also makes it a prison of sorts for those not possessing a portal key. Thus, many call Sigil "The Cage". Though Sigil is commonly held to be positioned atop the infinitely tall Spire, some argue that this is impossible since the planes are infinite in all dimensions, and therefore there can never truly be a center to any of them, let alone all of them; thus, Sigil is of no special importance. Curiously, from the Outlands one can see Sigil atop the supposedly infinite Spire.
Nevertheless, Sigil contains an innumerable number of portals: any bounded opening (a doorway, an arch, a barrel hoop, a picture frame) could possibly be a portal to another plane, or to another point in Sigil itself. Thus, the city is a paradox: it touches all planes at once, yet ultimately belongs to none; from these characteristics it draws another of its names, "The City of Doors."
The ruler of Sigil is the mysterious Lady of Pain. The Lady is sometimes seen in Sigil as a floating, robed Lady with a face bearing a mantle of blades. The Lady does not concern herself with the laws of the city; she typically only interferes when something threatens the stability of Sigil itself. However, she is an entity of inscrutable motives, and often those who cross her path--even accidentally--are flayed to death or teleported to her hidden Mazes, lost forever.
Sigil is, theoretically, a completely neutral ground: no wars are waged here, no armies pass through. Furthermore, no deities can enter into Sigil; the Lady has barred them from the Cage. Of course, Sigil is hardly peaceful; with such a condensed population, consisting of everything from devas to demons, violence is common, usually befalling the foolhardy and incautious. Most natives of Sigil ("Cagers") are quite jaded as a result of living here.
Sigil is divided into six districts, called wards:
The Astral Plane is the plane of thought and memory, and of psychic energy; it is where gods go when they die or are forgotten (or, most likely, both). Some strange creatures, such as the tyrannous githyanki, use the petrified corpses of dead gods as floating fortresses. The Astral Plane is unique in that it is infinitesimal instead of infinite; there is no space or time here, though both catch up with a visitor to the plane once it leaves. The souls of the newly dead pass through here on their way to the afterlife from the Material Planes.
The material plane, also known as the Prime Material, holds many worlds not unlike our own, most of which have no direct connection with Planescape save the occasional portal or magical spell. People coming from such worlds are often treated as clueless inferiors by the planar elitists who dwell in Sigil and other planes.
The Ethereal is often likened to an ocean, but rather than water it is a sea of boundless possibility. It consists of two parts: a Border Ethereal which connects to each Material and Inner Plane, and the Deep Ethereal which acts as the incubator to many potential demiplanes and other proto-magical realms. From the Border Ethereal, you can see a misty grey-scale version of the plane from which you are traveling; however, each plane is only connected to its own Border Ethereal, which means inter-planar travel necessitates entering the Deep Ethereal and then exiting into the destination plane's own Border. Many demiplanes, such as that which houses the Ravenloft setting, can be found in the Deep Ethereal; most demiplanes are born here, and many fade back into nothingness here.
The Inner Planes are the building blocks of the multiverse, the elements and energies from which all else is created. They are arranged in a sphere; at the top is the Positive Material Plane, the plane of life and vibrant energy, and at the bottom the Negative Material Plane, the plane of death, entropy, stagnation and the forces that bind and empower the undead. The four classical elements (Air and Earth, Fire and Water) lie on the "equator" of the "globe" that makes up the Inner Planes. Where the true elements touch each other the Para-Elements are formed. Where Air meets Fire, Smoke is formed; Fire and Earth yield Magma; Earth and Water create Ooze; and Air plus Water equals Ice. Where the Positive and Negative planes touch the Elemental Planes, Quasi-Elements are formed. None of the opposing planes touch one another, as they cancel each other out violently, particularly in the case of Positive and Negative Energy.
The Plane of Shadow was once a demiplane but recently became a full-fledged transitive plane. It is, as one would expect, an empty plane of darkness, where shadows are cast without any source of light. The Shadow Plane connects to other planes not only through portals but also through darkness; this is especially true on the Material Plane, where a monster or mage might use the shadows as a form of teleportation.
Demiplanes are minor planes, most of which are artificial. Demiplanes are commonly created by wizards and demigods. Naturally-occurring demiplanes are rare; most such demiplanes are actually fragments of other planes that have been somehow split apart from their parent plane. Demiplanes are often constructed to resemble the Material Plane, though a few--mostly those created by non-humans--are quite alien.
The Factions are the philosophically-derived power groups of the planes. In particular, the factions controlled, until recently, the political climate of the city of Sigil. Each of the factions is based around one particular belief system; many of the factions' beliefs make them enemies where their other goals and actions might have made them allies. There are fifteen factions in total, per a decree of the Lady of Pain; any additional factions emerging would be subject to her wrath. The fifteen factions are:
Portals, conduits and gates are all openings leading from one location to another; some lead to locations in the same plane, others to different planes entirely. Although the three terms are often used interchangeably, there are notable distinctions. Portals are bounded by pre-existing openings (usually doors and arches); if that opening is destroyed, the portal is as well. Portals also require portal keys to open; a key is usually a physical object, but it can also be an action or a state of being. Naturally occurring portals will often appear at random, especially in Sigil; some portals only exist for a brief period of time, or shift from one location to another. Conduits are also naturally occurring, but they are natural phenomena, the planar equivalent of whirlpools and tornados. Conduits are only known to occur in the Astral and Ethereal Planes. A type of conduit known as a color pool is a common gateway from the Astral Plane to the Outer Planes. A vortex is a link from a Prime Material world to the Inner Planes, which begin in areas of intense concentration of some element (e.g., the heart of a volcano might be a vortex to the Plane of Fire). Gates are portals that are not bounded by physical apertures; gates are rare, and usually appear as a result of magical spells and rare planar phenomena. Lastly, planar bleeding occurs when regions of two planes coexist; such phenomena are usually short-lived, and disastrous for their environs.
Planescape is an expansion of the idea presented in the Advanced Dungeons & Dragons Dungeon Master's Guide (First Edition) and the original Manual of the Planes. It in turn inspired the computer role-playing game and the revised edition of the Manual of the Planes; material from the latter has been incorporated into the Dungeons & Dragons Dungeon Master's Guide (Third Edition Revised).The Planes
Outer Planes
Sigil
Astral Plane
Material Plane
Ethereal Plane
Inner Planes
The Shadow Plane
Demiplanes
The Factions
Portals, Conduits and Gates
Published Material