The Great Wheel Cosmology

The Great Wheel cosmology is the prevalent cosmological model in which the world of Zedrelon was said to exist during the mid-9th century AT and the post-First War era.
The Great Wheel was described as a complex, comparatively cosmopolitan place in which the gods of many worlds and pantheons mingled, the beliefs of many faiths and peoples bleeding together in a set of Outer Planes shaped predominantly by the polar forces of Law, Chaos, Good, and Evil. Thus it was that Lliira and the Greek goddess Hecate could feud over the love of the Sumerian god Enki and work at a pleasure palace operated by the Aztec gods Xochipilli and Tlazoteotl.

Manifestation

Overview

The Great Wheel cosmology consists of a series of somewhat concentric spheres. In the center is the Prime Material plane containing the phlogiston with the crystal spheres and worlds within, surrounded by an Ethereal plane (misty realms of proto-matter). Outside of the Prime and Ethereal are the Inner planes, also called the Elemental planes, which had their own structure based on a sphere. Then came the Astral plane which connected the worlds in the Prime Material plane to each other (bypassing the phlogiston) and also to the last sphere, the Outer planes. The Prime Material plane touches both the Astral plane and its Ethereal plane, though these planes do not touch one another. The Outer planes, also called the Planes of Power, are 16 planes arranged in a circle (the Great Wheel) defined mainly by alignment and surrounding a 17th neutral plane known as the Outlands.

The Inner Planes

The Inner planes are the six major elemental planes (Fire, Earth, Water, Air, Positive Energy, Negative Energy), the four para-elemental planes (Smoke, Ice, Ooze, Magma), and the eight quasi-elemental planes (Lightning, Steam, Radiance, Minerals, Vacuum, Salt, Ash, Dust).
They could be thought of as being on the surface of a sphere with Positive Energy at the north pole, Negative Energy at the south pole and Fire, Earth, Water, and Air on the equator, equidistant from each other. The para-elemental planes were found on the equator between the boundaries of Fire, Earth, Water and Air (Magma was between Fire and Earth, for example).
Four of the quasi-elemental planes are found between the boundaries of Positive Energy and the four elements (Steam is between Positive Energy and Water, for example). And the other four quasi-elemental planes are between Negative Energy and the four elements (Vacuum is between Negative Energy and Air, for example). The Inner planes are surrounded by the Ethereal planes, which connected them to the Prime Material planes.

The four elemental planes, Air, Fire, Earth, and Water, plus the two energy planes, Positive Energy and Negative Energy, make up the six major Inner planes. Between each pair of adjacent elemental planes was a para-elemental plane. The eight quasi-elemental planes were found between the four elemental planes and the two energy planes. The four para- and eight quasi-elemental planes add up to twelve minor planes for a total of eighteen Inner planes. Each plane is made up of primarily one type of matter but pieces of other Inner planes may have found their way in (or been brought in by powerful forces). One thing all Inner planes had in common was they were all hostile to lifeforms from outside, particularly the Prime Material Planes.

Fundamental Planes

Surrounding the Prime and the parallel planes are the two fundamental planes: the Astral Sea and the Elemental Chaos. The fundamental planes, according to common theory, are the basis from which all the other planes, including the Prime and its echoes, were formed. Whether or not this is true is unsure.
The fundamental planes were also the homes of the astral dominions and elemental realms, smaller planes within the Astral Sea and Elemental Chaos. Only a few of these were known to the inhabitants of Toril, however, and it is believed that if one traveled far enough through the infinite spaces of the fundamental planes it might be possible to locate the dominions and realms of entirely different pantheons worshiped by beings from other material planes, though such a journey would be unimaginably lengthy.

The Astral Sea

The Astral Sea is a great silvery void that contained the homes of the gods and which was considered the realm of pure thought and creativity, where thoughts and feelings became reality, if only for a moment. It was through this power that gods shaped the cosmos according to their will and the astral dominions were forged.

The Elemental Chaos

The Elemental Chaos is a vast and infinite sea of tempestuous matter and energy in which the cycle of creation and destruction continued without end. It was from the essence of the chaos that the world was first formed, tempered though it was by the divine essences of the Astral Sea.

Parallel Planes

Parallel planes are strange reflections of the Prime, formed by the will of the primordials in the age of creation who gathered what they considered to be imperfections of the Prime and created new planes from them. Parallel planes are the closest of the planes to the Prime and resembled it geographically, though they were very distinct in their own manner.
The parallel planes are:

The Feywild

A vibrant twin of the Prime, the Feywild is the home of the fey and is filled with vitality and powerful arcane magic.

The Shadowfell

A dark and twisted echo of the Prime formed by Hela from the Plane of Shadow and part of the Negative Energy Plane, the Shadowfell is the home of shadow creatures as well as much of the undead that filled the planes.

Demiplanes

Demiplanes are planes of finite extent found within an Ethereal plane. They may have been the creations of extremely powerful wizards, technologists, or demigods or they may have been created when a large glob of proto-matter began to pull away from its Ethereal plane and achieved separation. Demiplanes might eventually collapse in on themselves, re-merge with its parent Ethereal, or merge with an Inner plane or Prime Material plane. Each demiplane had its own rules regarding gravity, vision, magic, and material make-up.

Ethereal Plane

In the Great Wheel model each Prime Material plane has its own Ethereal plane in which nothing was solid, including living creatures, their possessions, weapons and armor. All metal becomes ethereal metal, flesh became ethereal flesh, stone to ethereal stone, etc.
The Ethereal touches its Prime at all points and binds it to the Inner planes but the connections are not abrupt transitions like to/from the Astral plane but rather a gradual process in what was called the Border Ethereal.
If you consider the Ethereal to be an ocean, a person in the Border Ethereal would be standing in the surf near the shore, invisible to those on land but able to see them and their surroundings dimly. One could travel along the shore, staying in the surf, and step back on land at some other point, or move away from the shore toward the Deep Ethereal. A curtain of vaporous color marked the transition between the Border Ethereal and the Deep Ethereal and each plane had its own color. After traversing the Deep Ethereal and reaching the curtain of the correct color, you would pass through to the Border Ethereal of your destination plane.
By use of magic (or the natural ability that some creatures were thought to have, like phase spiders) one could fade into the Ethereal and travel at will. Gravity gave a sense of up and down, but movement in any direction was equally easy and objects released hovered in place. The proto-matter swirling about the Ethereal plane could be used to create demi-planes, either through natural fluctuations in the medium or by the actions of powerful beings. The known Demi-planes at the time were the Demi-Plane of Shadow, the Demi-Plane of Time, the Demi-Plane of Electro-Magnetism and the possibly legendary Demi-Plane of Imprisonment.

Astral Plane

The Astral plane is described as infinite nothingness interrupted only by small islands of material that broke off from their native planes and occasional spinning columns of astral conduits (called wormholes or gates and resembling water spouts from a storm at sea).
The Astral plane connected the various Prime Material Planes with each other and to the first layers of all the Outer planes. Travel in the Astral was usually accomplished by spell, psionics, device, or color pool and involved leaving your physical body behind while your astral self traveled to your destination. During this transit, a nearly unbreakable silver cord connected your astral self back to your physical body.

Outer Planes

The Outer planes are organized according to alignment, which is most easily visualized as a wheel with spokes radiating from the center of true neutrality.
In the diagram below, planes associated with Good (sometimes called the Upper planes) are found above the line running from Nirvana to Limbo, and Evil-associated planes (sometimes called the Lower planes) are below. Planes associated with Law are found to the left of the line running from Elysium to Hades, and Chaos-associated planes to the right. Many of the Outer planes were divided into layers—infinite sub-regions that metaphysically overlapped the other layers of the same plane.

When you arrive at another plane, a new physical body manifested out of local materials. Wormholes linked specific places in the Outer planes to each other and to fixed locations in the Primes—quicker but likely more dangerous because your physical body was transported directly to a terminus that might be inhospitable or guarded.