There are many, many worlds. Each world’s sun is surrounded by an impenetrable celestial sphere. Each celestial sphere, casually known as a sphere, is floating in a chaotic energy of aether which is surrounded by the Void. Inside a celestial sphere the aether is an invisible, flowing psychic medium which can affect the reality of each celestial sphere. Outside and between each sphere it is an organic matrix of visible rivers and paths between each sphere.
The majority of the population of all the worlds are oblivious to the aether and other worlds, but there are those who build ships which use the aether to both travel within a celestial sphere and through the aether rivers between the worlds itself. These men and women who travel the the aether are named aethernauts. Vessels that travel the Aether, whether they travel between worlds or not, are called aetherships. Those who travel between spheres are sometimes called celestials.
Some vessels are powered by science, some by magic, some by the sheer will of psionic creatures, and strangely some are a combination of two or more methods! There is no wrong or inferior way to travel the aether.
This contraction/combination works well for Aether-Adventure, Aether-Aperture, or even Aether-Literature.
In the nothingness of the Void between spheres are rivers of glowing celestial Aether. This ghostly substance has positive and even healing properties. These rivers ebb and flow between spheres, and travel so fast that it only takes an aethership a few weeks to pass through the Void. The Void is a semi-conscious being, much greater than a simple god. It only wishes only to consume everything to join its nothingness. The aether cuts through it and keeps it at bay as long as it travels fast and is in large quantities. Those who lose their way on the celestial rivers between the vast Void are doomed to be consumed. The Void cannot pass through the a celestial sphere’s walls.
The Void has contacted and employs several slave species which have their own agenda, but serve the Void’s purpose for consumption. One well known race that has ‘sold its soul’ to the Void are the Qrai (pronounced “Cry”). These potent psychic creatures feed on the very life forces of sentient creatures.
The Void is not the villain in the setting, it is a negative force of nature. It has no conscious goal to thwart the protagonists and will consume indiscriminately between its slaves and others alike.
These vessels come in a myriad of designs and sizes. Whether powered by psionics, magic, technology, or a mix, the ship has the following traits: an aethereal shield that contains a bubble of atmosphere protecting the vessel from the vacuum of space, radiation, and debris and the ability to move along the aether. These bubbles, or aethereal fields, also provide gravity to those within. Some ships are retrofitted from sailing ships, and some are specifically built to be travelling the aether not designed to set down on or water or land.
Eons ago, an ancient, extinct race of aethernauts, sometimes called “The Aencients” explored many of the spheres inhabited by developing man and other sentient beings. In ancient times, the aether between spheres was a chaotic, confusing place. These Aencients placed quasi-magical teleportation apertures, Aethertures, which used shortcuts through the aether to connect and instantly travel to other Aethertures in the same, or even other celestial spheres.
The default setting is a familiar setting loosely based off Earth in the late 1800's. It's not our historical world, but an alternate, steampunk-style history where magic was real and the Egyptians held much power. Aetherships and Aethertures are virtually unknown, but some historical hint towards their existence. There are two Aethertures on Aerth. One hasn't been found, and the other is in the control and forgotten by a very powerful organization.
Within Aerth's Crystal Sphere there's Vaenus, and Maers, both of which have habitable, but exotic climates. Aetherships pass quietly through space and in between the planets unbeknownst to the people of Aerth. Some dedicated astronomers have noticed these moving lights, but since they're not as predictable as the rest of the celestial bodies, they're lost and forgotten blamed on equipment failure or atmospheric effects.