Structure of the Multiverse
by George Hrabovsky1) Prime Plane
By mutual agreement between the Immortals (despite not infrequent attempts by just about everyone to bend the rules) there is no direct Immortal power used in the Prime without the direct approval of all hierarchs.
This is the plane where most of the game takes place.
2) Ethereal Plane
This surrounds the Prime Plane and is either the place where psychic powers manifest, or it is a lower realm of the astral plane. Nobody is sure which is true. One thing is known by some scholars, it is here that psychic powers work through this plane. Access between the ethereal and Prime planes is easy for psychics and trivial for magicians.
2-a) Elemental Planes
These are planes that exist within the Ethereal and it is thought that they were created by powerful beings called elemental kings. Each kingdom is thought to have existed as a separate plane at one time, but over the ages similar planes have collected together to form four primary planes: Air, Earth, Fire, and Water. The individual kingdoms now constitute worlds within these planes, and pure dilute forms of the element form the spaces between the worlds.
2-b) The Dreamworld
This is where the consciousness goes when someone has a dream. Skilled dreamers can manipulate the reality of this world and influence the dreams of others. Powerful dreamers can cast out emanations of dream into other planes, thus influencing reality. This is NOT the dimension of Nightmare!
2-c) Limbo
A shadowy place where people go when they die. It is possible to rescue people from this land. This may only be accomplished by powerful magicians, priests, or shaman. Servants of the Immortals come here to bring the faithful dead to their final resting places (usually the home planes of Immortals). Greater undead can be found here too, looking for servants and slaves...
3) The Astral Plane
This is the plane where magic manifests itself. It is always night here, with stars filling the sky. Each star is a wormhole through which the traveller can cross into outer planes.
4) The Outer Planes
These are the homes and/or creation of the Immortals. They are diverse and the rules of existence in them may be very different from that of the Prime Plane.
5) The Dimension of Nightmare
Beyond the 11 dimensions of the Prime Dimension is the Dimension of Nightmare. Nightmare also has 11 dimensions but they are not all the same as those of the Prime. Beings from Nightmare are horrifying to behold, the only known beings are the Malfera, the Nuckalavee, The Brain Collectors, and the Burrowers (corrupted by Thanatos, and thanks to Andrew for reminding me of them).
6) The Vortex Dimension
Nothing is known of this even by the Immortals, other than it is the place where Blackballs come from.