I did mine somewhat backward for the current campaign. I had an idea for how a multiverse could be set up and worked backwards from there until I got to the planet/plane the campaign takes place on. Most of it will probably never come up for the players, but I had fun working it out:
Cosmology of the Multiverse
This explanation for the structure of the local multiverse is the one most favored by the various druidic sects. Clerics tend to prefer a more deity-centric version, while Warlocks tend to favor a version that places their patron at the top of the celestial pecking order. Wizards as a whole do not favor any specific model of the universe, instead various wizardly organizations and individual sages create highly detailed models that reflect their individual views on how the multiverse functions. The only thing that all the models created by wizards have in common is that they all place arcane magic as the predominant force in the universe.
The druids use a “top down” model of the multiverse, with the Source at the top and the Terminus at the bottom. All of creation lies between them, created by the interplay of energies between the two.
The Source
Also known as the “Wellspring of Creation” this is the source of all magic, all life, and indeed all of creation. It continually radiates the energy of creation that set the universe in motion. No creature, not even a deity, can survive the overwhelming Primal energies that radiate from the Source. They must first be filtered through several planes of reality (planes mostly composed of pure energy) before they can be safely absorbed by a god, and the gods can then convert the Primal energies into Divine magic and imbue their mortal followers with a fraction of that magic in return for their worship. (Worship allows a god to increase their capacity for storing and using Primal energy, which in turn increases their power.)
The Terminus
The counterpart of the Source, it is the Final Destination, the Sphere of Ultimate Annihilation, the End of All Things. Everything the Source creates, the Terminus devours. It is the tension between the two that caused the infinite planes of reality to come into being. Without the constant Entropic energies from the Terminus the Primal energies emitted by the Source would never break down into the mundane matter and various forms of elemental energy that make up the all of existence. Magic itself is a byproduct of the decay of Primal energy.
The Great Cycle
New planes of reality are continuously created as the Primal energies released by the Source decay. While they may seem eternal to a mortal, even the various planes of existence go through a cycle of birth, growth, maturity, decay, and eventual destruction as the entire plane falls into the Terminus and is consumed. It may take millions of years, but it is the eventual fate of every plane of existence.
The Interstitial Planes
These are the planes that make up the spaces between other planes. Generally you must travel through one of them in order to get to another plane, although powerful spells and permanent gates can make it a very short journey.
The Astral Plane
Connects the High Heavens to the Mortal Realms, and the Mortal Realms to the Lower Realms. It can be reached via Astral Projection and Dreamwalking, it is also the plane that Teleportation type spells send you through (very briefly) before reaching your destination.
Physical travel to the Astral Plane itself is difficult, and even then the plane seems to reject physical matter. Travelers report being pulled towards the nearest Color Pool (a one way gate to a Mortal Realm) with increasing force and speed the longer they stay there. If they do reach their chosen destination quickly they will find themselves ejected into a random Mortal Realm.
Despite this, rumors persist of a race of mortals that make their home in the Astral Plane - the legendary Githyanki warriors with their Silver Greatswords, the greatest of whom ride red dragons into battle.
The Ethereal Plane
Connects all the Inner Planes to each other, and the Inner Planes to the rest of the Mortal Realms. While it is possible to use the Astral Plane to travel directly to the Inner Planes, it is much faster to use the Ethereal Plane instead. (i.e. From the High Heavens you would take the Astral Plane, exit into a Mortal Realm, then access the Ethereal Plane from that Mortal Realm and use the Ethereal Plane to reach the Plane of Elemental Fire. Trying to go directly to the Plane of Elemental Fire via the Astral Plane would take weeks or months longer.) Why this is so is a poorly understood aspect of planar mechanics.
The Shadowrealm
Connects all the Mortal Realms to each other, but does not allow access to the Celestial Realms or Inner Planes. May allow access to the upper layers of the Lower Realms, but rarely on purpose. Accessed through shadows and dark places, naturally. Generally requires magic to open a portal, but there are naturally occurring portals as well. They are usually formed on moonless nights and stumbled into by lost mortals.
The Shadowrealm has shadowy copies of structures in the Mortal Realm and is inhabited by various creatures of shadow and darkness.
The Plane of Mirrors
Similar to the Shadowrealm, it connects the various Mortal Realms to each other. It also connects to a few mirror-themed Inner Planes of Order, and to a few powerful magic mirrors in the High Heavens and Lower Realms. It is accessed via specially enchanted mirrors or hard to find rituals, making it much harder to reach than the Shadowrealm. While it has far fewer inhabitants than the Shadowrealm, travel is not without its risks. Mirror doppelgangers have a chance to appear and try to murder their Mortal counterparts.
The Celestial Realms
The planes closest to the Source where the various deities make their home. Only gods and beings protected by deity level magic can survive here, as the intense Primal energies overwhelm any lesser being, causing them to burn up from the inside. As new planes are created near the Source, eventually the older planes are pushed “down” and are no longer filled with raw Primal energy. When that happens the gods simply move their homes to one of the relatively young planes that are now closer to the Source.
The High Heavens
The planes that were formerly the homes of the gods themselves, these now serve as the Afterlife for all the gods faithful. While not constantly exposed to raw Primal energy, these planes still have a high level primal energy. Mortal beings cannot survive more than a few days or weeks here. Instead, when a mortal being dies the bit of Primal energy that makes up their soul is called to the High Heavens by their god, and the faithful soul is transformed into a celestial being. Most of the faithful souls are content to live out their afterlife in relative peace and happiness as they serve their respective deities.
Eventually these planes of the High Heavens are also pushed farther and farther away from the Source as new planes are created. Most of the faithful souls follow their gods during the celestial migrations to new planes nearer the source, but some choose to end their immortal afterlife and become mortals again as the plane they are on descends into the mortal realms. These brave souls become the seeds of new nations and races at the dawn of an age of legendary heroes in the new mortal realm they now inhabit.
The Mortal Realms
The vast majority of the planes of existence are categorized as “Mortal Realms”. Eventually a plane in the High Heavens is pushed far enough from the Source that Celestial beings cannot live there. Celestial beings and deific Avatars may visit the Mortal Realms when summoned or sent on a mission, but they cannot stay indefinitely or their Primal energies will fade and they will become mortal. Some choose this fate on purpose, giving up immortality in order to stay with a mortal lover, or because they have grown tired of the political maneuverings in the Celestial Realms. Similarly, mortal beings can visit the High Heavens, but cannot stay there or they will eventually be killed by exposure to the Primal energies that permeate them.
The Inner Planes or the Elemental Planes
While technically part of the Mortal Realms, the Inner Planes do not have the relatively equal mix of elemental forces that make up the rest of the Mortal Realms. Instead they are heavily biased towards one elemental force or another. There is no one single plane of each type, but instead a multitude of planes that share a similar elemental bias and more easily form dimensional gateways to each other via the Ethereal Plane. Thus there are many different Planes of Fire, or Planes of Chaos.
They are referred to as “Inner Planes” because (rightly or wrongly) they are seen as making up the “core” of the Multiverse of Mortal Realms, and that core is surrounded by the rest of the Mortal Realms. The Ethereal Plane is seen as the plane that connects all the Inner Planes together, and in turn connects them to the rest of the Mortal Realms.
Examples of Elemental Planes
-Earth
-Fire
-Water
-Air
-Light
-Dark
-Order
-Chaos
-Many others
The Far Realms
The Far Realms are left over from a previous cycle of the multiverse. The Primal energies of the Source are harmful to their inhabitants. For the most part the alien deities who reside in these realms are content to slumber, waiting for the next age of the multiverse where they might find conditions more to their liking.
Occasionally sentients from the mortal realms unwittingly make contact with lesser beings from the far realms while dreaming (or tripping on powerful drugs or magic). This tends to drive them a bit insane and they usually become obsessed with finding ways of waking the alien gods early. This rarely works out well for anyone involved.
The Lower Realms
This far from the Source, the Entropic energies of the Terminus begin to dominate. These planes have begun their slow decline into oblivion. Resources are scarce, magic is harder to access, and the sun and stars have begun to fade, if they ever existed in that particular plane of reality. The ambient Entropic energy of the Lower Realms causes non-magical items from the Mortal Realms to weaken and decay far more rapidly than is natural. This effect becomes more powerful the closer you come to the Terminus.
Home of the lesser Fiends, and the fiendish animals and plants that have managed to adapt to the harsh realm their world has become.
The creatures of the Lower Realms, the Hells, and the Infinite Planes of the Abyss have adapted to the energies of the Terminus, making their flesh tougher and more resilient to normal weapons, with many being resistant to magic as well. However, this comes with a price. The natives of the Lower Realms cannot live easily in the Mortal Realms because of the Entropic energies that permeate their flesh. The lack of ambient Entropic energy in the Mortal Realms causes the Fiends to bleed out Entropic energy into the environment, sickening them and tainting the local area with their Infernal essence. This usually manifests first as a sulphur and brimstone stench, but can lead to many other effects depending on the power of the Fiend and the amount of time they spend in the Mortal Realms.
If they stay for more than a short time they will try to make themselves more comfortable by making their lair more like the environment of the lower planes. They increase the local Entropy levels by various methods - driving mortals mad and keeping them as pets, surrounding themselves with undead (a side effect of necromantic magic and undead in general is that they constantly “leak” Entropic energy), etc.
Natives of the lower realms traffic in souls due to the lack of magical resources (indeed resources of any kind) this close to the Terminus. The soul of every living being contains a spark of Primal energy. Various techniques can be used to induce the Primal spark to release a fraction of it’s energy and once filtered through a mortal soul they can use the energy to sustain themselves, extend their lives, power their magic, etc. Their entire society runs on energy harvested from souls. The souls can also be used to strengthen materials so that they can better resist the Entropic energies that infuse the Lower Realms. Items infused with soul energy (i.e. Primal energy) in this way are referred to as “Soulforged” or “Hellforged”.
Souls native to the Lower Realms have the least value (i.e. produce the least energy) and souls native to the Celestial Realms have the most (but are also the most difficult to acquire and harvest). Mortal Realm souls are the base unit of currency, with the souls of animals and unwilling sacrifices being the least valuable. Souls gained via a bargain are sought after because a living soul gives up much more energy before becoming useless. The most highly sought after of all are those living souls who willing make a pact with a creature of the lower planes in return for power. This gives the pact giver a link to the mortal realms, and energy continuously flows down that link to the pact giver, giving them a renewable source of energy, one that produces greater and greater amounts as their warlock grows in power. (The abilities that the warlock gains represent only a minor drain on the energy that the pact giver gains from the pact.)
The Courts of Hell
Below the Lower Realms are the Hells. The most famous are the 9 Hells ruled by the Arch-Devil Asmodeus, but there are many others ruled by various other devils and fiends.
The Infinite Planes of the Abyss
The layers closest to the Terminus that can still sustain life, they are almost exclusively inhabited by Demons.
The Demons constantly war with the Devils for land and resources. While the demons have more actual territory in terms of the number of planes they inhabit or control, these planes are all very resource poor. The devils have far fewer planes, but they have many more resources compared to the Planes of the Abyss. Similarly, the Demonic Hordes vastly outnumber the Legions of Hell, but the Legions are better trained and equipped. Plus the devils are much more disciplined and devious than the demons, and constantly outmaneuver the Hordes so that they can rarely bring their full might to bear.
