Realmspace

Realmspace represents the entirety of known existence, believed by planar sages to be a self contained universe housed within a crystal sphere. It is strongly theorized that Realmspace is but one of many spheres, each containing their own entire universe, and that travel between them could be made possible with sufficient knowledge, resources, and preparation.

Cosmography

Realmspace is made up of an assortment of dimensions known as Planes. These Planes are organized by their location (though such a concept transcends the physics of three dimensions) and their individual composition. Planes fall into one of four categories: Transitional Planes, Inner Planes, Outer Planes, and Material Planes.

Material Planes

The Material Planes (sometimes referred to as Real Space) are those most easily understood, as most mortal beings exist within them. While the size of a material plane can vary widely, with some potentially infinitely large, the most common variety contains a singular sun and a handful of planets that orbit it. The Prime Material Plane is one such plane, containing Astora along with several other unknown bodies orbiting an orange sun.
Many Material Planes exist in Realmspace, and each is layered atop one another in a higher dimension of existence. As they exist apart from each other, one would need a means of planar travel or divination to perceive or travel to another material plane. One common means of explaining this layering is by describing each plane as a two dimensional space rather than three. Various Material Planes would then be stacked atop one another like sheets of paper in a book. From the perception of those within each plane, the others do not exist, and traveling to another plane would require leaving the contained two dimensions of the plane you are currently in.
Material Planes are unique amongst Realmspace as they are the only planes connected to all of the Transitional Planes. This has led to most planar scholars considering them to the the "center" of a crystal sphere.

Transitional Planes

Transitional Planes exist between other planes, serving as both a barrier and pathway between them. The four known Transitional Planes are the Ethereal Plane, the Astral Sea, the Feywild, and the Shadowfell. All of the Transitional Planes are connected to the prime Material Planes, though none of them border each other or are accessible to each other directly, making the Material Planes the only central connection point to all of the Transitional Planes.
The Ethereal Plane serves as the barrier between the material plane and the Inner Planes, as well as countless Demiplanes. The Ethereal Plane serves as a particularly weak barrier between the planes, and there exist many direct connection points, or nodes between the planes contained within it.
The Astral Sea contains all of the Outer Planes, and serves as the main form of travel between them and the Material Planes. The barrier of the crystal sphere containing Realmspace is at its weakest in the Astral Sea, marking it as the primary ingress and egress point for those wishing to enter or leave Realmspace.
The Feywild and Shadowfell are both special transition planes, as each serves as a node between the Prime Material Plane and their respective Inner Planes. It is not known if these two planes are exclusive to the Prime Material Plane, or if they are similarly layered and have versions connecting to all Material Planes. The Feywild serves as a connection point between the Plane of Life and the Prime Material Plane, while the Shadowfell links the Prime Material Plane to the Plane of Death. Each transition plane acts as a reflection of the Prime Material Plane, though distorted and changed by the energies of their respective Inner Planes.

Origins of the Feywild and Shadowfell:
It is believed by many theologians that the Primordials crafted these two realms to serve in the creation of the Material Planes. The Material Planes were crafted using energies from the Inner Planes, including the elemental, quasi-elemental, and para-elemental planes, which could all be linked directly to the material plane with ease. The positive and negative energy planes (that is the Plane of Life and the Plane of Death) could not be so directly linked without destroying the material plane, and so the Feywild and Shadowfell were created to serve as a muted link through which they could still channel their energies in their act of creation.

Transitional Entities

There are several features of the Outer Planes that serve as connection points between them, but do not rise to the definition of planes themselves. Such features include the following:

• Oceanus: A plane spanning river connecting many of the upper Outer Planes to each other.
• Mount Olympus: An inter-planar mountain believed to span several Outer Planes as well as the Prime Material Plane.
• Styx: A plane spanning river passing through many of the lower Outer Planes.
• The Infinite Staircase: A network of pathways linking to many of the Material Planes and Outer Planes.
• Yggdrasil: An inter-planar tree with roots and branches connecting many Outer Planes as well as Material Planes.
• The Living Gate: A gateway to the Far Realm, a universe outside of Realmspace and home to all manner of eldritch horror.

Inner Planes

The Inner Planes are contained within the Ethereal Plane, and are all linked to one another to form a spherical lattice. These planes can only be reached from the Material Planes, either directly through connection nodes, or indirectly through the Ethereal Plane.

Outer Planes

The Outer Planes are all contained within the Astral Plane. connections between them are few, and those that exist are either closely guarded secrets, heavily fortified and guarded, or hotly contested and fought over. The Outer Planes are aligned radially around the Concordant Domain of the Outlands, giving rise to the great wheel cosmological model by which the Outer Planes are percieved.