World Axis cosmology
The World Axis was a cosmological model created after the dramatic changes to the nature of the multiverse caused by The Spellplaguellplague of 1385 DR, and quickly became the most widely accepted model during the late years of the 14th century DR and for most of the 15th century.
This structure, which resembled a sphere split into two axes—hence the name—resulted from the Spellplague and its effects upon the local multiverse, which included the untethering of the astral dominions and the creation (or possible return) of the Elemental Chaos. As a result of the changes wrought by the disaster, the formerly prominent World Tree model was abandoned in favor of the World Axis, which most scholars believed more accurately reflected the current structure of the multiverse.
Basic Structure
The World Axis was composed primarily of the fundamental and parallel planes. "Above" and "below," the fundamental planes encircled the rest of the cosmology, forming hemispheres in the conceptual model, though since both the Astral Sea and the Elemental Chaos were infinite in size, this was mostly metaphorical. To the "left" and the "right" of the Prime Material Plane, which laid at the center of this cosmology, sit the parallel planes, which floated in sync with the Prime. Together these planes formed the World Axis, other planes lying outside of the model and having an indistinct cosmological placement.
Prime Material Plane
Also called the natural world, mortal world, or simply the Prime, this plane laid at the center of the World Axis. While changed, the Prime continued to exist in spite of the changes wrought by the Spellplague, stable and at the center of the vast axes that binded the planes together. The Prime was where Toril and its inhabitants were located and remained the primary focus of the gods. The Prime had no single dominant force, unlike most planes, and was connected to the other planes through The Shadowfell and, to a lesser extent, The Astral Sea and The Elemental Chaos
Fundamental Planes
Surrounding the Prime and the parallel planes were the two fundamental planes: the Astral Sea and the Elemental Chaos. The fundamental planes, according to common theory, were the basis from which all the other planes, including the Prime and its echoes, were formed. Whether or not this was true is unsure[speculation] and until the Spellplague, the Elemental Chaos was not believed to have existed, or at least not since the splitting of Abeir-Toril into two worlds.
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 two fundamental planes are as follows:
The Astral Sea
The Astral Sea was 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
Nonexistent before the Spellplague, or at least not since the sundering of Abeir-Toril, the Elemental Chaos was 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 were 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 were the closest of the planes to the Prime and resembled it geographically, though they were very distinct in their own manner.
The parallel planes were:
The Feywild
A vibrant twin of the Prime, the Feywild was the home of the fey and was filled with vitality and powerful arcane magic.
The Shadowfell
A dark and twisted echo of the Prime formed by Shar from the Plane of Shadow and part of the Negative Energy Plane, the Shadowfell was the home of shadow creatures as well as much of the undead that filled the planes.