Cosmology

=== The Multi-verse ===
The multi-verse is all of the varied planes of existence, all bound together by the Astral Sea. Each plane contains its own planets, stars and space, though we have yet to find a plane where travel between planets is possible. New planes are growing and expanding all the time as well as older or unstable planes collapsing.
The planet that the peninsula of Athesia is on is part of a contested plane. While often referred to as the Prime Material Plane by its inhabitants, it is not the Prime Material Plane from Planescape and similar D&D lore because we don't follow that setting, and there isn't a specific primary plane (each plane is considered home to their respective inhabitants).
The nature of a plane is dictated by the deities that rule there. When many different deities contest a plane, as it the case for our Prime Material, the laws of reality become less consistent. Deities compete with each other for the belief of mortals, deities with more worshippers become more dominant in that plane of existence, and are more able to exert their own influence over the nature of that plane.
At the centre of the multi-verse lies the city of Sigil.
=== The Astral Sea ===
The Astral Sea is the lifeblood of the multi-verse. It runs through all known planes of existence and it's energies are the birthplace of deities. All deities are (or were once) astral beings. When deities need (or desire) to travel through planes of existence, they mostly do so through the Astral Sea. There are also mortal means of inter-planar astral travel but most involve Sigil (a place where deities cannot go) mortal bodies cannot survive the energies of the sea.
=== What is Sigil? ===
Sigil looks like a torus (giant floating maths doughnut) with a city built on the inside curve. The city itself has thousands of doors, that, with the right kind of key can be used to access any plane in the multi-verse (though each door will only go to one specific place). Deities cannot enter Sigil, they are barred from entry by the city's ruler. Accessing Sigil from Athesia is a very rare event and considering how hazardous the city is, it is generally not advised.
=== What is the Rainbow Shell? ===
There are a few structures referred to in the history of Athesia as the Rainbow Shell. The two most talked of are the Rainbow Shell around the planet and the Greater Rainbow Shell around the multi-verse.
The Rainbow Shell is a literal physical construct, it is a thin layer of solidified energy. It encapsulates the entire planet that Athesia is on, like a layer of atmosphere. It looks like the oily sheen of a bubble, which is where its name comes from. While invisible at night, the Rainbow Shell can be seen with the naked eye on bright, sunny, cloudless days as slight rainbow tinge in subtle patterns in the sky, though it is still very difficult to see without ocular devices.
The Greater Rainbow Shell surrounds the entire multi-verse and was specifically created by a conglomerate of deities to separate the multi-verse from the Far Realms. This is what makes travelling to the Far Realms so difficult, and mostly keeps the eldritch horrors of Illithids and Beholders at bay.
The Rainbow Shell was designed to shut out the influence of the Astral Sea. However it only partially performs these functions due to damage it has sustained. Astral beings are unable to cross the Rainbow Shell of their own volition, but if enough force is applied, external or internal, they can be brought through whole or in-part (often creating fractured vessels of power such as angels and avatars). Energy can pass through the Rainbow Shell but only in limited quantities and Astral energy is very rare inside the shell, with one notable exception being souls. It is also easier for energy and matter to leave the Rainbow Shell than to return.
=== Deities and Demigods ===
The difference between the two is primarily location. Deities are god level beings that exist outside of the Rainbow Shell and demigods are those that exist inside. While deities are more powerful beings than demigods, power levels between them are effectively irrelevant as a deity's influence on Athesia is filtered and restricted by the shell, whereas a demigod can apply its influence directly. The term demigod also refers to fractured aspects of a greater deity, but they perform the same functionally while inside the shell.
Examples:
Pelor, Bahamut and Anubis are all deities
Loki, Corellius, Larethia and the Ancestor Gods are all demigods
=== Afterlives ===
Afterlives exist as planes of existence, and are also all deity's realms. This includes the rare afterlife realms where entrance isn't directly tied to worship of specific deity (like Valhalla for God-Cursers, it is still the realm of the Aesir deities).