Planescape, simply put, is about the multiverse. Leave your 'prime' world behind and set foot in the many alternative planes of existence. Transit the ethereal to the inner planes, where the purest form of raw elements are housing vast cities and realms of efreeti, djinni, and far more exotic elementals.
Or pass through the astral plane - the silver void - to end up on the outer planes, the great wheel. This is the realm of the afterlife, the great playground of philosophy. Take any idea or belief from a prime world on heaven, hell, nirvana, primordial chaos, the happy hunting grounds and the prisons of tartarus, and you'll find its equivalent here. There's even those who claim that its those actual ideas and beliefs that sprung them into being in the first place. Realms, planes, even gods, existing by grace of their believers.
No wonder the belief is so hotly contested. At the centre of the outlands, hovering atop a tall spire, is Sigil, the city of doors. From here, pathways - portals - lead to all over the multiverse. Sigil is home to the kriegstanz, the cold war-like state of conflict between the many factions that divvy up the city. There's those that believe in order, such as the Harmonium patrolling the streets, or the Guvners that tend to the city's administration. There's the Fated, that believe it is every man's right to take what he can. The believer's of the source, who believe in the self-made god, the Athar who are convinced all gods are frauds, mere con-men. The bleak cabal believe in nothing, just the senselessness of it all, while the Sensates believe that the multiverse is there to be experienced. The list goes on.
