The Planes - touching and parallel to the material world. Plane shift will work
The Underworld - shadowy realm of the past, the dead, and lost. Source of
power for Necromancy. Anything that has ever been is here. To find things of
the distant past you must travel deeper into the underworld. Shades of the
dead exist here, at every moment of their lives. Most are unresponsive and
distracted.
The Overworld - bright, hazy realm of possibilities. Source of power for Illusions
and Conjurations, and Divinations. The deeper one travels into the Overworld
the more hazy and unreal are the visions.
The Near Worlds
Planets and Moons
Primal Realms - elementals, plants, animals
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The Far Worlds - the outer planes in D&D terms. Need Astral Travel
Abyss: The abyss is actually the void or dark between the stars
Olympus
Hell
Arboria
etc.
We have changed how the planes of existence work. The world of the Celephais Campaign, called Gaia by the Arcadians (most Player Characters), is a planet orbiting a sun with other planets. Most planes are worlds orbiting other suns, detailed later in this document. Planes are parallel and coterminous to the material world. There are a few magical pocket realms that are also tied to the material world, but they are finite and limited. All of these planes can be reached via plane shift, Gate or Astral Travel.
The Ethereal plane is the material plane, just "out of phase". If you are in the current time/space and Ethereal, but in an area where no sunlight has touched for millennia, you might have hit a pocket realm of shadow where Undead exist. It is the sole province of the divine to be able to exist in the Overworld, Underworld and material world simultaneously.
The Underworld is actually the world of the past - fixed by time and slowly fading. What once existed exists forever in the past/Underworld. Nothing really lives - they just exist, perhaps fading into Oblivion. The lower/deeper/farther you travel into the Underworld the farther back in time is seen until all fades and you reach the wall of Oblivion. The areas near the Wall of Oblivion is remarkably like the Negative Energy Plane. Entire sections of the Underworld are sealed away from the prying eyes and hands of explorers.
Undead tap this plane for their eternal and damned existance. No undead can forever be destroyed, as necromancers from later ages can summon them from the Underworld to exist again.
At the other end is the Overworld, a realm of endless potential and possibility. All possibilities are here, the higher/deeper/farther you travel the weirder and more vibrant it gets until you reach the wall of Infinity.
Plane shift allows you to enter the Overworld, Underworld or Etherial planes. If the caster is at the proper place in the material world, he can also enter a pocket realm.
Outsiders reside on distant worlds warmed by other stars. The planes of Celestial translate very well into seven planets orbiting a cluster of suns, such as the Pleides. The Celestials are formed of light and are highly evolved and moralistic beings, capable of transporting themselves accross the void to answer summoning spells. The Devils and Diabolic races were originally celestials, but were banished for their crimes and are made of flame. Demons are those remnents of creation, or things so diabolic, that they were banished to the darkness between the starts. Some demons, such as the demon princes, have returned to despoil planets akin to their home planes. All Outsiders use a power similiar to Astral Projection to cross the void.
Celestials are formed from light
The Devils of hell were once Celestials that rebelled against the laws and government of the Celestials.
Demons are those remnents left over from ancient, evil realms, or malignent bits of matter that gained sentience bewtween the stars
Elementals are tenuous, etherial spirits that manifest through a large amount of their element. There are no elemental planes as such, but naturally occuring elements are present in the world. Large concentrations of one element and magic can create a pocket plane that resembles and elemental plane.
The Geniekind are a race created by the Protogenoi, or first gods, similiar to the creation of the Elves. Unlike the elves, the Genie are not in harmony with the living world but with the elements. There are four tribes of Genie: Djinn (Air), Efreet (Fire), Marid (Water), and Jann (Earth). Most have close ties to the Titans and were exiled after the Olympian ascendance. Those that betrayed their tribe and sided with the Olympians remain on the floating island of Serranian, which drifts above the Arcadian Sea.
The Fae come from a distant world, steeped in so much magic that there really is no natural laws at all - only magical laws. These magical laws can be confusing and disconcerting non Fae. Unlike Outsiders such as the Celestials or Infernals, the Fae have crossed the void in Material form using Gates and settled in magic rich areas of Terra. These magic rich areas are also desired by the Elves but long ago the Fae and Elves have achieved an uneasy truce.