Great Wheel

The Great wheel was a cosmological model, which encompassed each of the planes of existence.

Structure
The Great Wheel was an arrangement of the planes, represented as spheres, on two axis, a vertical axis (representing good and evil) and a horizontal axis (representing law and chaos), with the Inner Planes: the Material Plane, the Elemental Planes, the Elemental Chaos and the Transitive Planes at the centre.

Inner Planes
The Inner Planes made up the Centre of the Great Wheel.

The Ethereal Plane
The Ethereal Plane greatly resembled the material plane, although nothing there was solid. The Ethereal Plane could be accessed from any point on the Material Plane and thus could be used for efficient, incorporeal travel. Such travel was often used by creatures such as Phase Spiders in order to travel unseen. Such travel greatly resembled teleportation to an observer on the Material Plane. It was on the Ethereal Plane that Ghosts and other incorporeal undead had the most capacity for interacting with the living.

The Astral Plane
The Astral Plane was a great sea of infinite, silvery nothingness, interrupted only by small islands of materials broken off from other planes. Being a Plane of Thought, Psionic Creatures such as Mind Flayers and Githyanki held great power, with the latter being a dominating presence, sailing the plane in great ships. The Astral plane could be magically accessed by means such as an Astral Projection spell, leaving one's physical body behind while one's Astral self arrived at the destination. Creatures undergoing such Astral travel were tethered to their physical forms by a nearly unbreakable Silver Cord. Many wormholes to various outer planes existed on the Astral Plane, so it was often used as a means of travel between worlds.

The Elemental Plane of Air
The Elemental Plane of Air resembled an endless blue sky on a summer day, dotted with floating islands of solid material, which were often deliberately brought into the plane by intelligent beings and usually inhabited. Where the plane of air bordered the Elemental Plane of Water, the Frostfell could be found, as well as the Great Conflagration where it met the Plane of Fire. The Elemental Plane of Air was inhabited by Aarakocra, Djinnis, Smoke and Ice Mephits, Elementals, as well as many powerful air Archomentals, such as Yan-C-Bin and the Wind Dukes of Aaqa and countless natural airborne creatures.

The Elemental Plane of Water
The Elemental Plane of Water was a bottomless, trackless sea, the depths of which were filled with the domains of Marids and other aquatic races, as well as all manner of underwater life, while the unfathomably deep portion, so far from the surface that light did not reach it, was inhabited by Krakens and other monstrosities. The surface of the endless sea was dotted with man-made islands, (mostly ships and rafts lashed together to create platforms), while its endless seas were plied by ships from countless worlds (which were occasionally transported to the Plane of Water during intense storms at sea). The Isle of Dread, a primeval, monster-infested island, often shifted between the material plane and the Plane of Water. The Frostfell marked where the Plane of Water met the Elemental Plane of Air and the Swamps of Oblivion was the intersection between the Plane of Water and the Plane of Earth. The Elemental plane of water was home to many Merfolk, Tritons, Sahuagin, Aquatic Elves, Elementals, as well as Olhydra, a Princess of Elemental Evil.

The Elemental Plane of Earth
The Elemental Plane of Earth was an endless network of mountains and caverns, the walls studded with gemstones and inhabited by burrowing creatures of all sorts. The Elemental Plane of Earth was ruled by the greedy Dao, who lived in the City of Jewels and sought to enslave most creatures they came across. Many other elemental creatures roamed the Plane of Earth, including Earth Elementals, Dust, Mud, and Magma Mephits, and Gargoyles. The Elemental Plane of Earth was also home to Ogremoch, the Prince of Elemental Evil Earth. The Swamps of Oblivion marked where the Plane of Earth met the Elemental Plane of Water, and the Fountains of Creation bordered the Elemental Plane of Fire.

The Elemental Plane of Fire
The Elemental Plane of Fire was a hellish realm, filled with flames of all sorts. Land in the Plane of Fire was composed of odd, loosely packed elemental flame, with the seas filled with a more liquid form of similar fire. The Plane of Fire was home to the City of Brass, the legendary city of the Efreeti, famous across the multiverse for its great brass domes and fine palaces. Notorious slavers, the Efreeti were served by the Salamanders and, occasionally, the Azer. The borders of the Plane of Fire were the Great Conflagration, where it met the Plane of Air, and the Fountains of Creation, where it met the Plane of Earth. Few creatures other than Fire Elementals could survive on the Plane of Fire, which was inhabited by Magmin, Fire Giants, Steam and Magma Mephits, Red, Brass, and Gold Dragons, Hell Hounds, and Devils. The Elemental Plane of Fire was also home to Imix, a Prince of Elemental Evil.

Plane of Smoke
The Plane of Smoke, also known as the Great Conflagration was a hellish world where the gales of the elemental plane of air fused with the floating cinders of the plane of fire. Filled with firestorms and noxious gas, few visited the plane of smoke, which made its few solid areas excellent hiding spots. The Plane of Smoke is home to Djinn, Efreeti, Mephits, and Smoke Para Elementals.

Plane of Ooze
The Elemental Plane of Ooze, usually referred to as the Swamp of Oblivion, was an endless stretch of mud, dotted with gnarled trees, stinging vines, and stagnant lakes. The Swamp of Oblivion had few inhabitants, save for voracious swarms of mosquitoes, Para-Elementals, Mephits, and scattered humanoids, who lived in settlements suspended above the muck. It was said that anything that sank into the Plane of Ooze would not resurface for at least a century. For that reason, desperate souls occasionally visited the Swamp of Oblivion to cast powerful artifacts into the muck.

Plane of Magma
The original home of the Azers, the Plane of Magma was also known as the Fountains of Creation. A ring of rocky peaks surrounded a great caldera, that was the fiery heart of the plane. The Fountain's skies were prowled by Red Dragons, and Fire Giants and Efreeti sailed through the magma on brass ships.

Plane of Ice
The plane of Ice, usually referred to as the Frostfell, was a great glacier, with caverns cut deep into the ice and a frigid sea beneath it. Due to the scarcity of resources on the plane, its inhabitants were in a constant struggle, where only the strongest could even survive. Most visitors to the Frostfell stayed in the air, braving the cutting winds and raging blizzards to avoid the Yetis, Remorhazes, and terrifying Frost Salamanders that prowled the glaciers.

The Feywild
The Feywild was a land of everlasting twilight, full of music, light, wonder, and death. The Feywild was home to the Seelie and Unseelie fey, with the former dwelling in elaborate courts and the latter stalking their prey through the untamed magical wilderness. An echo of the Material Plane, the Feywild was its more flamboyant, spectacular counterpart. The Feywild was inhabited by Elves, Centaurs, Faerie Dragons, Treants, Unicorns, Blink Dogs, Hags, Giants, Ogres, Goblins, Blights, Dryads, Satyrs, and countless other Fey.

The Shadowfell
The Shadowfell was a bleak echo of the Material Plane, devoid of colour and life. A sunless, misty realm, The Shadowfell was a darkness that hated the light. Like the Feywild, it was a counterpart to the Material Plane and mirrored locations on that realm, albeit as dark, twisted versions of them. Sadness and despair pervaded the Shadowfell and anyone who stayed there for long was at risk of going mad or sinking into depression. Deep within the Shadowfell were the Domains of Dread, within which were imprisoned beings of great evil. The Shadowfell was haunted by various undead, Shadow Dragons, and Sorrowsworn. It was also home to the Shadar-Kai elves.

Outer Planes
The Outer Planes were the sixteen worlds on the edge of the great wheel, with each corresponding to a non-neutral alignment. The Upper Planes (The Planes stationed on the top half of the wheel) represented goodness, while the Lower Planes represented evil. The planes on the left side of the wheel were associated with law, while those on the right were associated with chaos. As the Outer Planes could be reshaped by the powerful beings who lived there, the concept of distance was absolutely meaningless. The perceptible terrain could seem quite small, yet stretch on to infinity, if the powers of the Plane so desired it. Many of the Outer Planes had multiple layers (smaller realms within the greater plane that were usually imagined and depicted as stacked on top of one another). Gateways into the plane from the outside usually opened into the first of these layers (the highest or lowest, depending on the plane).

The River Styx
The lower planes were connected via the River Styx, a nauseating river of filth that could cripple the mind of any who touched its waters. The main branch of the Styx cut a direct path through the highest layers of these planes. but smaller tributaries flowed through every layer. Yugoloths, and other fell plied the waters of the Styx in sinister boats, willing to ferry from plane to plane anyone who could afford their services.

The Infinite Staircase
The Infinite Staircase