The world on which the Moss-Wren Isles are located is often described using a model termed Disc Cosmology. In Disc Cosmology, three discs each containing four main planes intersect on a single point: the Prime Plane, also called the Material or Prime Material Plane. The planes of different discs intersect with each other inconsistently, disrupting each other as they do so.
Prime/Material Plane
The Prime Plane is the confluence of all other planes and is made up of the substance of each, whether physically, mentally, or relationally. The inhabitants of the Prime Plane often cannot cross onto any of the planar discs without some form of magic allowing them to step over the boundary between them. Inhabitants of the Prime Plane are often termed mortals.
There are four main segments of the Prime Plane, with little overlap between the four, though travel and trade between the segments does occur, most commonly between the Overbright and the Underdark. These inner segments are more populous and larger than the outer segments (the Skylands and the Deeplands).
Skylands
Floating above the surface of the Overbright, taller even than the tallest of mountain peaks, lie the islands of the Skylands. There are tales of flying cities, of hills that uprooted themselves to wander and find somewhere else to settle, of towns that vanished off the face of the world only to descend years later. Many stories of the Skylands center on pieces of the Overbright that lift into the air, buoyed by magical forces rarely understood by the scholars of the Skylands. However, while some pieces come back to the ground over time, many have orbited the Overbright for longer than mortal memory.
Most inhabitants of the Skylands are capable of flight, whether by wings or enchantment. Trade between individual pieces of the Skylands is accomplished through flight, and trade between the Skylands and the Overbright mostly takes place in areas where mountain peaks make it easier for Overbright residents to reach the heights of the Skylands.
Overbright
The Overbright, often called the Surface or the World, is the area on the outside of the world of the Moss-Wren Isles. There is a certain depth and size at which caves are considered Overbright rather than Underdark, though this is not widely agreed upon—the most agreed upon metric is environment and inhabitants—and there is a certain height at which floating things are considered the Overbright rather than the Skylands, largely agreed to be the alpine treeline of mountains, or the peaks thereof for smaller floating objects.
Most inhabitants of the Moss-Wren Isles live in the Overbright and so it is seen as the default for the setting. The people of the Overbright are detailed in the main body of the People of Moss-Wren. Most trade in the Overbright is accomplished through overland travel or sea travel.
Underdark
The Underdark is a sprawling mass of caves and caverns that runs under the surface of the world of the Moss-Wren Isles, lying beneath the Overbright, connected to it by tunnels and passages that are often extremely dangerous to traverse. The Underdark is permeated by a low level of magical radiation which allows plants to grow and fungi to thrive. Nonetheless, it is seen by Overbright residents as a barren and dangerous place. The Underdark is relatively stable in relation to the shifting, flowing pockets of air that make up the Deeplands.
The underground denizens of the Moss-Wren Isles most often live in the Underdark, and many are comparable to the people of the Overbright. These inhabitants are detailed in the Underdark section of the People of Moss-Wren. Most trade in the Underdark happens by caravans that travel through the tunnels.
Deeplands
Much like sections of the Overbright lift into the sky to join the Skylands, sections of the Underdark sometimes sink into the ebb and flow of the deep pockets of air that make up the Deeplands. There are no consistent paths under the Underdark—the earth flows like water, not quite magma but nigh unto it in parts. There are tales of cities vanishing, never to be heard of again. Unlike the occasional return of an Overbright area after the land voyages to the Skylands, the sections of the Underdark that vanish into the Deeplands never return.
Little is known about the inhabitands of the Deeplands. Trade is almost impossible between even the deepest sections of the Underdark and the Deeplands.
Physical Disc
The four elemental planes are found on the Physical Disc. It gained its name through the belief that the four elements of these planes overlap to form the material aspect of the Prime Plane. Each of the elemental planes overlaps with the adjacent planes to form a paraelemental semiplane—a section of reality that simultaneously exists on two elemental planes without being a separate plane in and of itself. The paraelemental semiplanes are distinct from demiplanes in that they are not distinct from full planes. They provide a bridge which can be used to cross between elemental planes without using any magic, though the elemental inhabitants of the elemental planes are often unable to cross as they struggle to survive in a place without their element.
Elementals are the main inhabitants of the elemental planes. They do not like to leave their planes, but they can be drawn from them by high concentrations of their element on any of the other planes. They most usually are attracted to the Prime Plane because of their constant overlap with it.
Most of the elemental planes and paraelemental semiplanes are uninhabitable to those who are not elementals because they are made purely out of things of the element they are. However, there are always beings who carve out their own areas—sometimes in demiplanes and sometimes within the planes themselves—to live.
Fire
The elemental plane of Fire is a mass of burning, flame with no origin. Prime Plane scholars often debate what is being burned to create the flames, with some thinking that the plane burns reality itself and others saying that while fire on the Prime Plane needs fuel, there are no such bounds in a plane made purely of flame.
Fire elementals of many shapes and sizes live on the elemental plane of Fire. They sometimes cross into other planes when enough flame is concentrated in one place without magic, causing even more fire at their arrival.
Molten Earth
The paraelemental semiplane of Molten Earth occupies the edge between the elemental planes of Fire and Earth. It is a sea of shifting lava and melted rock, cooling as it draws closer to the elemental plane of Earth.
Earth
The elemental plane of Earth is solid rock, dirt, and gravel with no pockets of air for mortal creatures. The plane is a shifting mass, with different types of earth mixing with one another as elemental factions move. Occasional pockets of air that are considered to be parts of the elemental plane of Earth have been discovered to be distinct demiplanes, though few would argue that they are the domain of any but the earth elementals, and thus many consider them to be a part of the elemental plane of Earth despite the demiplanar distinction. Movement through the elemental plane of Earth is accomplished through burrowing and flow of materials.
Earth elementals do not have much differentiation from one another, mixing in the stone with consciousnesses more akin to a hivemind than a group of individuals. Few earth elementals leave the elemental plane of Earth without magical interference, though those that do inhabit stones or mountains and do not do much to affect the world surrounding them.
Sludge
The paraelemental semiplane of Sludge or Mud occupies the edge between the elemental planes of Earth and Water. It is looser than the plane of Earth, with stone, dirt, and gravel diffusing into a muck that slowly thins from almost immovable to free-flowing as it approaches the elemental plane of Water.
Water
The elemental plane of Water is a large expanse of fresh, salt, and brackish water, with no surface and no bottom. Creatures who do not need air or who can breathe in water abound, with some creating areas in which they can thrive, though these areas drift through the plane with nothing to anchor them.
Water elementals are the primary denizens of the plane, and though they are in some ways similar to earth elementals with their habit of intermingling and occasionally sharing minds, they are far from a hivemind, with many having individual whims and separating from other elementals regularly. Water elementals occasionally cross between planes to inhabit bodies of water.
Water elementals also are not the only residents of their plane, as other creatures also find they can make their home there. Many aberrations who find themselves in the elemental plane of Water find that they are able to make themselves homes, with a faction of aboleths in particular finding the plane a favourable habitat. Many swimming creatures may accidentally swim into the plane of Water, and so there is always a non-zero number of sharks in the water.
Mist
The paraelemental semiplane of Mist occupies the edge between the elemental planes of Water and Air. It starts as a heavy fog that disperses into the air as it reaches the elemental plane of Air.
Air
The elemental plane of Air is a place with no ground—only an expanse of the insubstantial not-nothing of air. Gravity is unpredictable, and things may fall in many different directions. Movement through the elemental plane of Air is almost universally experienced as falling at terminal velocity; there is little drifting or hovering or even flying without very specific tools or magic.
Air elementals are often drawn away from the plane by storms and gales. They are much more individual than other elementals, rarely fully mingling. Air elementals do not fall in the plane of Air, and rather take amusement at the plight of non-elemental creatures that find themselves in the plight of unchanging gravity.
Fume
The paraelemental semiplane of Fume occupies the edge between the elemental planes of Air and Fire. It starts as a faint smoke and heat and grows into the conflagration of the elemental plane of Fire. Motes of flame drift through the air along the clearest boundary between elemental planes that there is.
Mental Disc
The four planes of the Mental Disc are made up of emotion and degrees of connection. The substance of these planes takes on a physical aspect in the realms they contain, but on the Prime Plane, they are seen in emotional states, thoughts, and internal relationships of both creatures and sapient mortals. Unlike the planes of the Physical Disc, each of the four planes of the Mental Disc are made up of different sections influenced by the bordering mental planes. These planar sections blur together slightly and are somewhat fused, but are still recognizable as different planes.
Each of the planes of the Mental Disc has an ideological opposite in the plane furthest from it. The Heavens are the opposite of the Fiendlands, much in the way that the Feywild is the opposite of the Shadowfell. Mortals can often survive on the planes of the Mental Disc physiologically, but the planes are difficult for the minds of mortals to stay whole within.
High concentrations of the concepts embodied by the beings within the planes of the Mental Disc can summon the creatures that embody those concepts to any other plane. This happens more regularly with fey than with other sorts of beings.
Heavens
The Heavens are the home of creatures of connection and relational bonds. This is a plane largely associated with what mortals may term “good” as a moral judgement, but as moral judgements are subjective, many scholars consider this a misinterpretation of the material of the Heavens. Rather, the Heavens made of a collective state of being and care for things outside of an individual. While this manifests most often on the Prime Plane as what others consider good, it can often manifest as the nurture of things that cause destruction.
Celestials are the beings of the Heavens. Each celestial has a goal, a person, an order, or a cause outside of itself that it is wholly devoted to by nature of its very being. Celestials sometimes embody different kinds of bonds through their devotion, and by their nature they cannot break the bond that they are made of.
There are three sections of the Heavens.
(Passionate Heavens)
The (passionate heavens), also called the Passionate Heavens, borders on the Feywild. In the (passionate heavens), the bonds of celestials are driven by an emotional connection, and the celestials are more likely to act to chase a particular emotion for or in relation to their bond.
Compassion is often seen as the driving emotion of the (passionate heavens), though sympathy and empathy are also highly associated with this segment. Celestials from the (passionate heavens) are prone to ignoring reason or to let their vision be clouded by emotions. This intensifies the closer to the Feywild a celestial originated from.
Pure Heavens
The Pure Heavens, the main body of the Heavens and balanced between the emotion of the Feywild and the apathy of the Shadowfell. Celestials from the Pure Heavens balance their emotional ties to their bonds with the
(Apathetic Heavens)
The (Apathetic heavens), also called the Apathetic Heavens, borders on the Shadowfell. In the (apathetic heavens), the bonds of celestials are more often dictated by frank fact or dispassionate acceptance, sometimes characterised as logic. Celestials are more likely to act without much feeling about what they are doing or about their bond itself.
Cold benevolence is often seen as the driving characteristic of the (apathetic heavens), and mortals unfamiliar with the Broken Disc sometimes mistakenly attribute the concept of Law or benevolent order to the celestials of the (apathetic heavens). Celestials from the (apathetic heavens) are prone to hyper-rationalising their actions and failing to consider the feelings of their bond. This intensifies the closer to the Shadowfell a celestial originated from.
Feywild
The Feywild is the plane of emotion. It is made of