Cosmology
The world of Tellus is a rocky Earth-like planet orbited by five small moons and illuminated by three suns. I've already written a bit about that back in January. But what about the metaphysics of the Moon Shadows setting?
The world is shaped by four elements and the power of two primordial forces. The elements are fire, earth, air, and water. The forces are the Light above and the Darkness below. The realms of Light and Darkness are the provinces of the gods, angels, and demons. Although it would seem to be a clear cut case of "black and white", it's not as simple as it first appears. Light is the source of life and Darkness is the source of death, but they aren't good or evil. Light is mostly good and Darkness is mostly bad, but that "mostly" is closer to two-thirds than the 99.99% most people would assume. (Of course, just because a demon of darkness isn't evil, that doesn't mean it won't hurt you. A grizzly bear isn't evil either, but it will still casually rip you apart and it's only motivated by simple hunger. Demons are driven by the categorical imperative of entropy to tear apart the universe.)
Imagine the material world as the center of an octahedron with one vertex pointing up and another down with four vertices around the center. (Or imagine a cube if that's easier to picture. The important thing is there are 6 directions away from the material world: up, down, and 4 sides.) The Elemental Realms are the four sides, Light is the top, and Darkness is the bottom. Everything exists somewhere among those six extreme poles. Let's start at the top and go down through the three main layers of reality: Luminous, Material, and Tenebrous.
Luminous
The Luminous Realms are the layers of reality between the material world (including the Elemental realms) and the Light.
Empyrean
The Empyrean realm is the theoretical greatest extreme of the Light. It is assumed to be the source of the Light, but nobody can be certain. Not even the gods can approach it. Some philosophers have said that if the gods had gods of their own, those "overgods" would dwell in the Empyrean. Others say that if you entered the Empyrean, the Light would burn away every part of you that was not Light and all that would remain would be pure life energy without any thought, feeling, or identity: a most brilliant annihilation. The Light is the source of vital energy, which descends through the Luminous realms to the physical world before being drained away by death.
Upper Astral
The Upper Astral is where the luminous realms do not directly touch the material world. This is the realm of celestial beings like angels. The Upper Astral is commonly called Heaven or the Heavens. Physical directions like north and south don't exist here. Instead, things are described in terms of which elemental realm is closer, such as fireward or waterward.
The gods of Light—Morrow, Verath, and Saphyra—as well as the primordial goddess, Dunaia the Wellspring, all dwell in the Heavens, insomuch as they can be said to dwell in any one place.
Near Astral
The Near Astral realm is where the Luminous realms overlap with the physical world. It is much like the material world—directions such as north, south, east, and west exist—but everything is much more alive. Plants are greener, flowers are brighter, and fruit is sweeter. It's not an Eden, though. Animals are bigger, faster, and stronger. If you sit still too long, moss will grow over you and plants will start to put roots into your skin.
The Near Astral realm is where half of Faerie is located. Astral Faerie can be physically entered (sometimes even accidentally) through faerie gates that open in liminal times and places, such as a sea shore during a dawn green flash or a river bank during the predawn blue hour. Astral Faerie is strongly associated with nature spirits like dryads, but it has its share of grimwyrd, the bizarre fae "bogeyman" creatures of cautionary tales used to threaten naughty children.
Material
The Material realms are the four Elemental realms and the Tellurian, the physical world, that sits among them where they overlap. The Material realms are where the primordial god Mahkeh the Weaver binds the elements together to make the physical world and weaves the Light into the elements to make living things.
Tellurian
The Tellurian is the physical world, which many D&D settings call "the prime material plane". This is where Tellus and its moons are.
Elemental Realms
The elemental realms of Earth, Air, Fire, and Water are arranged in the obvious opposite pairs. They overlap and intermingle to form the material world. Each is inhabited by elemental creatures and ruled by a godlike Elemental Sovereign. For all practical purposes, the Elemental Sovereigns are gods, but they are considered a separate species of divinity, much like the Greeks distinguished between the gods of Olympus and the titans who came before them. The elemental sovereigns were worshipped as a pantheon called the Court of Seasons or the Elemental Court, but that religion is now only popular among the high elves of Mynthnyssor who treat it more as a cultural tradition than a matter of faith. There are still elemental shrines scattered across the world, but most were established to venerate a particular hero or holy person and they are treated like local saints.
The elemental realms are the power source for arcane magic. Using the power of the elements once required summoning minor elemental spirits and coaxing them to do one's bidding, but the study of magic has become much more scientific with rote formulae and sacred geometry to open a direct conduit to the elemental realms. The religious aspects have faded away. However, there are small mystery cults dedicated to the sovereigns that still exist. And as always, if there is a type of powerful supernatural being like the greater elementals, then there is always a witch willing to trade favors with it or a pactbound foolish enough to bind themselves to it as a shortcut to power.
Earth
The elemental sovereign of earth is the implacable Grond, the personification of both the immovable object and the irresistible force. The element of earth is what gives material objects solidity, structure, and mass. Spells and attacks empowered by the elemental energy of earth deal force damage. Force damage is more than just the simple kinetic impact of a solid object: it's the relentless crushing force of gravity. Earth gives people more than their flesh and bones; it also grants resilience, stability, and focus.
Air
The elemental sovereign of air is the restless and untamable Shisura. The element of air gives material objects movement, conductivity, and volatility. Without air, things are motionless, noiseless, odorless lumps. Air is what gives people quickness of thought and action, curiosity to look into every nook and cranny, and insight to find loopholes and paths around obstacles, both physical and metaphorical. Spells and attacks empowered by air deal lightning damage. Earth gives a metal sword its hardness and keen edge, but the element of air makes it a good conductor.
Fire
The elemental sovereign of fire is the passionate and dangerous Karak-Akash. The element of fire gives material objects their warmth and reactivity. Without fire, things are inert and cold. Fire gives people their passion and drive and appetites. Without fire, people would starve to death for lack of hunger. Fire gives them the motivation to eat and reproduce, to create new things, build and expand. Fire is the ambition that always wants more. Obviously, spells and attacks empowered by fire deal fire damage.
Water
The elemental sovereign of water is the wily and mysterious BĹŤda. Water gives material things their ability to flex, melt, absorb, and flow. Water is the reason that a person's blood is able to flow through their veins: Water carries their Fire and Air through the Earth of their flesh to animate them. Water gives the mind empathy and intuition. Water provides for the flow of all things through time. Just as a torrential rain flows downhill and washes everything into the sea, Water sweeps the material world in an unrelenting flood away from the past and toward the future. Spells and attacks empowered by the energy of water usually do cold damage by slowing or stopping the movement of heat. Some powerful spells empowered by Water can even freeze things in time.
Tenebrous
The Tenebrous Realms are the layers of reality between the Material realms and the Darkness.
Near Dark
The Near Dark overlaps with the material world and it is the dark reflection of the Near Astral. Everything is gloomier and darker, but it is not without its own autumnal beauty. Two important places are located in the Near Dark.
Umbral Faerie
The surface world of the Near Dark is where the more sinister and frightening grimwyrd dwell. Like its bright Astral counterpart, Umbral Faerie can be reached through gates that open in liminal places but at darkening transitional times such as the blue hour of dusk or the summer solstice when daylight begins diminishing.
Fell Deep
The subterranean world of the Near Dark is known by various names—the underdark, the undercroft, the mythic underworld, and so on—but it's most commonly called the Fell Deep. The Fell Deep is a sinister place of darkness and strange radiations that glow and burn without warmth. It is inhabited by monstrous, unnatural alien things, but it is an important trade route traversed by the bravest or most foolish. The same endless network of Fell Deep caverns and tunnels is accessible through deep caves, not only on the world of Tellus, but also on the moons (and possibly even other planets). It is possible to walk into a cave on Tellus, pass through strange Fell Deep caverns, and emerge from a cave, crevasse, or fissure on one of the moons!
There isn't a constant flow of traffic through the Fell Deep that takes advantage of this, but it does happen somewhat regularly. Lunar expeditions through the Fell Deep are much less common than trade ships coming back to Europe from the East Indies during the 17th century, but more common than attempts to reach the south pole during the late 19th and early 20th centuries.
Netherworld
The Netherworld is where the tenebrous realms no longer directly touch the material world. This is the realm of fiendish beings like demons. The Netherworld is commonly called Hell or the Hells, but hells are more like countries or territories within the Netherworld. Like the Upper Astral, physical directions like north and south don't exist here and locations are described in terms of proximity to the elemental realms. Each of the four corners of the Underworld is home to a sovereign territory known as a Hell or Hell Realm with their own distinct species of demons, but the four great Hells do not rule the entirety of the Netherworld. There are many lesser kingdoms and countless individual fiends that are unaffiliated with any territory. The Netherworld is also home to the Gods of Darkness—Ayar, Vael, and Fel— and the primordial god, Cnaw the Devourer, dwell in the Netherworld.
Inferno
The Inferno is the most fiery hell in the Underworld. Infernal demons are distinguished as much by their smoke and brimstone as by their mammalian traits: hooves, horns, shaggy manes. They are primarily evil but there are less intelligent infernals who are neutral in exactly the same way that a rabid animal is neutral. The more powerful and intelligent infernal demons are lawful and build hierarchies of tyrannical authority to organize their unruly forces.
Labyrinth
At the most Earthward end of the Underworld, the Labyrinth is a complex citadel of massive stone cubes stacked to create endless corridors. Labyrinthine demons are distinguished by their insectoid traits: antennae, mandibles, stingers, chitinous exoskeletons. Whether they are evil is up for debate, but their absolute dedication to Law and their duty to destroy reality makes that point moot. Whether or not termites bear any real animosity towards you is not important when they are destroying your house. Labyrinthine demons are often clothed in bindings of some sort, such as being wrapped in leather straps, chains, or coils of rope.
Abyss
The Abyss is a fathomless ocean at the Waterward corner of the Underworld. Abyssal demons are aquatic with fish scales, fins, crab claws, or tentacles. Like the Inferno, the Abyss is dedicated to evil. The smaller and weaker demons tend to be very lawful and organized to such an extent that they seem to operate with one hivemind, like a large school of fish that turns and maneuvers in unison. The larger and more powerful abyssal demons tend to be more chaotic and selfish; they seek to consume and absorb lesser demons to enhance their own power rather than organize and coordinate their inferiors.
Void
The Void is a vast expanse of howling winds that whirl and toss flying islands about the Airward corner of the Underworld. The avian Void demons are dedicated to pure chaos. The smaller and weaker demons snatch and tear with their claws to terrify anything they can. The more powerful and clever demons work slowly and carefully to build elaborate networks of mortal servants; they weave increasingly elaborate webs of conspiracy until finally everything collapses, like setting up a complex arrangement of dominoes or building a house of cards just to enjoy watching it fall apart.
Far Dark
The Far Dark is the deepest depths of the tenebrous realms and the ultimate Darkness. It swallows everything like a black hole and there is nothing there but emptiness. All vital energy from the Light eventually reaches the Far Dark and is extinguished if it is not somehow caught in a loop of reincarnation or locked into a static afterlife to keep it in the higher realms. Nobody can be sure what really happens. Some say the Light is an infinite source of energy and the Dark is a bottomless sink that can absorb it forever. Others say that the light that reaches the Far Dark must be recycled somehow and returned to the Empyrean.