The Aether

The Aether, also called the Astral Sea, is the fathomless sea of raw magic which makes up most of reality. To human eyes, it appears as a bottomless sea of iridescent black liquid, motes of brilliant light swirling in its depths.
Once, reality was a mandala, perfectly ordered and perfectly symmetrical. Now it’s a mess. Following the First Death, the mandala was irreparably broken in an event known as the Shattering. During the Shattering, much of reality was lost or destroyed, leaving only a handful of splinters—islands of stable reality scattered across the Astral Sea. The human population of Orphan is confined almost entirely to these splinters.
Although the Aether is extremely dangerous, people nonetheless regularly traverse it. Now, a millennium after the signing of the Celestial Covenant, there are well-established trade routes between all major splinters. If you can pay your way, and you don’t mind the risk, there are ships that will take you anywhere you might need to go.

Navigating the Aether

Some very powerful beings are able to traverse the primordial chaos of the Aether under their own power. Others must employ vessels powered by arcane engines—so called aetherships. A person who travels on an aethership is called an aethernaut. Since aetherships are not designed to fly, only to sail the raw potentiality of the Aether, they are usually not particularly aerodynamic; most look less like ships and more like floating stone fortresses.
Aetherships often have some combat capabilities for defense. The Aether is home to all manner of dangers: ghosts, monsters, anomalies that’ll drive you insane if you look at them. Seasoned aethernauts deal well with that stuff, but passenger ships are often enclosed and windowless, in order to avoid harm to untrained passengers.
The Aether is a storm of raw potentiality, where things like distance and direction don’t work the way we’re used to. It defies mapping; while there are representations of it, they are by necessity complete abstractions, usually illustrating the routes between the splinters rather than the relative positions of the splinters themselves. Two connected splinters are not necessarily “closer” to one another than two unconnected splinters, because words like “near” and “far” are irrelevant in the Aether.
The geography of the Aether, simply put, is totally fucking unfathomable. So how do you navigate it?
You use a type of magical beacon called a pharos (plural pharoi). Pharoi act as fixed points in the inconstant Aether. Sequences of them are laid out for aetherships to follow, like blind bottom-feeders navigating by touch, along nonlinear space. Some pharoi are attached to small settlements in the Aether; such a settlement is called an aether station.
Aether journeys are long and arduous. Aether stations, being both safe havens and trade hubs, are vital fixtures in the Astral Sea. Many of them also house magical gateways between the Aether and local splinters.

Splinter Physics

Splinters are not literal islands; a splinter is a discrete plane of existence, separated from the Aether by opaque crystal walls. That’s one reason for the term: not only are splinters products of the Shattering, but they appear, from the outside, as vast shards of glowing crystal.
This crystal is actually a sort of protective shell, spontaneously generated by the clash between reality and unreality. Though this shell can be breached, doing so requires huge power. Most people prefer to just go through a pharos gate. Not only is it easier, there’s also less risk of being executed for it by the local authorities.
At first blush, the inside of each splinter looks like a whole world, but it is in fact a sliver of one. This sliver’s boundaries are marked by those same crystal walls; from the inside, they are invisible unless you’re very close, and whole landscapes are visible beyond them, forever out of reach. It’s like a video game—the parts of the world that were lost in the Shattering are now out of bounds. Even if you do break through the walls, there’s not more world beyond, just the Aether.
The sky of each splinter works according to this same logic—it’s there, even though it’s probably not literally there. The suns of the worlds which once were still appear in the splinters’ illusory skies, warming land and sea, which is a good thing, because otherwise everyone would be dead.
The thirteen known splinters are:

• The Andarine Sea: A great sea, with only a handful of small landmasses scattered in its midst.
• Caverndeep: A subterranean splinter. Great dark caverns and mirror-still lakes.
• Embermire: A landscape of overgrown jungle and treacherous swamps.
• Fel Dhavash: A barren landscape of rocky highlands and stretches of industrial desolation.
• The Ghostlands: Rolling plains of yellow grass, haunted by the wind and also by actual ghosts.
• Heaven: A perfect sphere of white stone hovering above Templegarden. It’s unknown if this sphere is Heaven itself or merely a gateway to it.
• Hell: A mysterious splinter presumably located somewhere underneath Fel Dhavash. There’s very little information about it, only spurious rumors.
• Hyylana: An arctic landscape of snowfields and frozen mountains.
• The Moranneon: A massive, primeval forest, home to all manner of strange and furtive creatures.
• The Orosca Desert: A desert splinter. Sand dunes as far as the eye can see, baking under the burning eye of the sun.
• The Radiant Waste: A parched, poisoned wasteland, irradiated by magical warfare.
• Templegarden: A large and temperate splinter, all rolling countryside and forested hills.
• Windreach: A sea of clouds, over which flying ships skim.

The Howling Void

In the beginning, there was no such thing as death. Then the Demiurge died, and Its death poisoned infinity with the concept of “end.”
This concept manifests physically as the Howling Void. The Howling Void is the antithesis of the Aether: where the Aether is potentiality, the Void is negation. It is a black hole, a roiling tumor of death energy at the very heart of the Aether. It is a force of nature; it is the greatest and final divinity. In the conflict between Heaven and Hell, it is the true winner.
The mechanics of death are as yet poorly understood. One theory has it that the Howling Void consumes the spirits of the dead, but most scholars believe that, upon the death of the body, the soul unravels and rejoins the black waters of the Astral Sea. There are beings that come from the Howling Void, though. They are not the shades of dead mortals, but spirit-beings in their own right—the servitors and actuaries of death. Their aims and motives are unknown.

A map of the Aether, showing the known splinters and the pharos routes linking them. Note that this map is necessarily an abstraction. Connected splinters are not necessarily "closer" to one another, it's just easier to render them as such.