Advice on creating a cosmology?

[GuyOnline]

I’m potentially planning on making my own homebrew cosmology for a homebrew campaign in the future. With that said, I’m not sure where exactly to start. Any advice or ideas/inspiration from anyone here? Thanks

[Accelerator]

All cosmologies are for one thing and one thing alone.

Making a story.

Wanna make a story of clear cut good and evil? Make good and evil gods, and an objective morality system. Let the big good be active and in your face. Let the magic system be morally good or neutral.

Wanna be gritty? Make your gods flawed, strange, or mostly absent. Make the cosmological history of your setting not so much 'good and evil' and more like '**** happens'. Magic is morally neutral, and quite often is Lovecraftian or malicious.

[Composer99]

So, this is all going to be kind of D&D-centric, so if that's not the system you're running, apologies in advance. Hopefully some of the advice will still be topical.

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Cosmology, D&D style, has four components (as I see it, anyway):

(1) The nature of the (Material Plane) universe beyond the campaign world. In most D&D games, this isn't very important.

(2) The structure of the multiverse. This is often an important part of many D&D games.

(3) The nature of the entities that dwell in the multiverse. Also normally pretty important.

(4) The nature of magic, whether in the Material Plane or the multiverse. The way magic ties into both the setting and the game mechanics makes it difficult to have magic in a custom setting work a lot differently than the game's default, unless you're willing to put in a bunch of work homebrewing mechanics to fit.

Other RPGs might emphasise different aspects or components of fantastic/pseudo-sci-fi cosmology. But if you're homebrewing a setting, whatever the RPG, I would consider thinking about those four components.

For instance, in some futuristic games, parallel or alternate dimensions might be "a thing", counting as "planes of existence" in the sense used above. You'd want to think about how those dimensions and the "prime" dimension in which the PCs spend most of their time (the equivalent of the Material Plane) fit together. In such games, component (1) above usually becomes much more important.

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With all that in mind, I would suggest going through these steps.

(1) When you're building a cosmology, start with the Material Plane, where the world in which the PCs will adventure. Ask yourself if you want the universe of the Material Plane to default to something more or less like our own universe, or something different. Only if you want something different do you need to actually figure out what that something is.

(2) Default D&D has a lot of other planes of existence. A lot. So I would recommend ditching all of them, and then either:
(a) Deciding on the "super-structure" of any multiverse cosmology you might want. The DMG has a few good examples on page 44. Then you decide which of the default planes - or adapted versions thereof or homebrew planes - you wish to incorporate into this structure.
(b) Decide which planes you wish to have present in the multiverse - perhaps going through the default list and yay-ing/nay-ing each one before coming up with adapted versions or new planes - and then decide how they will fit together into a larger structure.

If you end up with all of the default planes, well and good. But at least you had to systematically consider them.

That structure can matter, because it can modify the mechanics. If there is no Astral Plane, spells or items that interact with that plane, or creatures that inhabit it, must be modified or removed, for instance.

(3) If you adopt the default D&D planes, you probably don't need to modify their inhabitants - demons hail from the Abyss, and modrons from Mechanus, and so on. If you've made any changes, then you need to decide how that affects extraplanar creatures. For example, in the D&D 4E cosmology (the World Axis), Asmodeus is a god like any other, and, unless I'm much mistaken devils are his divine servants. In 5E terms, that could very well make them celestials.

(4) Among the inhabitants to consider are the ones that might serve as patrons or source of power for the PCs - gods, archdevils or demon princes, etc. etc. Who are these beings, what do they want with mortals, and why? I would start with a small list, and let it grow as the setting develops. To save some time and effort, you can ignore classes of beings that are either unlikely to come up in the campaign, or delay coming up with them until shortly before they become relevant.

(5) If you want to change how magic works, you need to either come up with a way to make that change compatible with the existing game mechanics, or modify the mechanics. This is true for D&D or any other game with magic (or equivalent capabilities). For instance, "magic works through quantum interactions in the Thaum Field" can probably work with D&D spell slots, despite being a kind of physics word salad, but "all magic is blood magic" probably requires some mechanical reworking.

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Last general advice: start small, but don't be afraid to let things grow as the setting develops.

[Silly Name]

The first step is figuring out what kind of theme/feel you want for your world, and how cosmology interacts with those concepts.

While looking at existing cosmologies can be a great and excellent source of inspiration (and you may find one that fits perfectly with your needs), having some fundamental ideas to start from is even better. For example, in one of my D&D campaign worlds I had decided there were multiple religions, and while not explicitely in contrast with each other, they appeared separate. So, I started off the idea that while clerical magic and summoning still works, nobody can just prove that their god(s) were more real or powerful than somebody's else, and thus the connection to and from the divine realms worked weird and direct travel and contact was impossible.

This later evolved into conceptualising travel between planes a la Spelljammer, done following "routes" through the Astral Sea, currents of energy and power that magical ships could follow, and the routes to divine realms being apparently damaged or non-functioning. Power still flowed from and to those planes, but you couldn't reach them.

If you wanted to look at a published setting, there's Theros for D&D 5e/MtG. Since that world is based off Greek mythology, the cosmology is less extensive than normal D&D and fits with the idea that you can physically enter the Underworld without having to use teleportation, can even escape from it. And the "Upper Planes", the dwelling of the gods and mythical creatures, is the starry sky itself, perfectly visible from the ground and fundamentally a tangible part of the world.

Lastly, you should figure how prominent the cosmology is going to be in your world and how important it will be to its stories. The City of Sigil is a planar metropolis, and thus it's important to have an established multiverse for it to work in - the City itself can be the scenario of the adventure, but it's just as often the starting point, the headquarters for the players who venture through the planes. So we need to know what the planes are, how travel between them works, how they relate to each other, etc.

But in a world like Greyhawk, the Great Wheel isn't nearly as relevant for your average adventure. Most stories will involve things happening on the Prime Material, from great wars and plots to exploring ruins of ancient empires and discovering powerful artifacts of ages past. Greyhawk's cosmology is there as needed, and the focus of the setting is on cities and nations and peoples of the physical world. The planes exist to provide an explanation for certain types of magic and creatures and where the gods are, but they're not a central theme.

And of course, you can flip everything on its head. Sometimes you just have a really cool idea for a cosmological structure and metaphysics of a world, and you start from there and let those informations shape the setting. Maybe you just read Plato and thought "hey, wouldn't it be cool if the world was divided between World of Ideas and World of Matter, with the former being structured in a pyramidal way?", and you decide to make this into a fantasy setting hinged on the influence of platonic ideals and the structure of the hyperuranion.

[Tvtyrant]

I’m potentially planning on making my own homebrew cosmology for a homebrew campaign in the future. With that said, I’m not sure where exactly to start. Any advice or ideas/inspiration from anyone here? Thanks

Cosmology only matters if you make it matter. Your world could be a giant animated Barbie House and as long as the players don't know it won't matter.

So I think "how important is cosmology to the setting" is the first question.

Then you have to put aside your writing chops and think in terms of players. If your cosmology is supposed to be involved heavily it has to open up options for players to do things, not shut them down. Dragonlance is the worst possible cosmology for an RPG, but it makes for passable Sword and Sorcery. Eberron's cosmology is great for RPGs, and is so uninspired that it might as well not appeal in a fantasy book.

Finally you need to have a distinction between cosmology as people in setting know it, and how it objectively works. For instance lots of RPG settings have gods that appear in setting as a wide variety ranging from good to evil, but from any distance they all become variations on a soul eating monster that divvy up the souls based on how the react to being thrown into a crappy nightmare world.

[brian 333]

As has been mentioned above, it's only important if you make it so. If your players never venture to the outer planes, their nature is not relevant.

Instead, I advocate importing or creating a basic pantheon without too much fluff so that starting clerics can have a patron, and then leave cosmology building for later because energy spent on the cosmos is energy not spent on what really matters: your next game night.

Those complete pantheons with developed cosmos are usually the work of professional writers who are paid to create them, and they are generally either overly complex to the point that players and DMs have nothing to add, or they are so trite that players and DMs ignore them.

As a player, which description would you rather for your level 1 cleric's patron?

Spoiler

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Gitah Modaq came to be in the time before time, and in the Realm of Potential which we know as Chaos she became aware of others. Some attacked her and tried to steal her essence while others clung to her in hopes she would protect them. As sides formed a great war took shape, with different groups vying for different goals.

As the war intensified Gitah Modaq unveiled her true form, and her radience blasted the battlefield. Many beings perished and many others were forced to reveal their power or flee the battlefield.

One of the survivors was Luna, whose face was burned when he gazed on her, but he cannot help but gaze on her. He heals and then burns again and again. His power of metric time was unveiled on the field of battle so that he could regenerate, and in this act the multiverse was created. The many entities trapped within shaped the cosmos and destroyed it again and again until this time. We do not know if it will be destroyed again or if the gods have achieved a true equilibrium this time, but the wise maintain both sides of the argument.

Or...

Spoiler

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Gitah Modaq is the goddess of the sun and the patron of agriculture and civilization. Her domains are fertility, protection, law, and light.

Her clerics pray for spells at dawn and make ritual offerings at dusk. Her churches are organized and there is a heirarchy in which succesful priests may climb the ladder to higher office.

The Lunites are an allied sect who worship the moon. Healing, magic, death, and time are their domains.

The second says nothing about the shape of the cosmos, but provides useful info, while the first is more fun to read but useless.

As a DM I always had limited time between sessions to build. In the rare moments when the next session was ready I would world-build. The next adventure is far more important than the world tree or wheel, or big bang, or first word or whatever.

[Max_Killjoy]

I guess when asked about cosmology, my first thought is not of a complex planar structure of existence, but rather the general meaning of the term.

https://en.wikipedia.org/wiki/Cosmology

And I do think having at least a general idea of the "deep history" and origin of a setting is important, as part of understanding the overall structure of the place, and how things like magic might fit into that.

[brian 333]

I guess when asked about cosmology, my first thought is not of a complex planar structure of existence, but rather the general meaning of the term.

https://en.wikipedia.org/wiki/Cosmology

And I do think having at least a general idea of the "deep history" and origin of a setting is important, as part of understanding the overall structure of the place, and how things like magic might fit into that.

I don't disagree. A basic structure is a good thing. But a fleshed out cosmos is time consuming, and it will only matter if characters need it to matter in game.

What do you want to happen when a character dies?

What can a dead character do while dead or recall if brought back?

How easy or difficult would it be to raise a fallen comrade?

These questions are important, and the DM should be ready to answer them.

What is the shape of the multiverse, and how does one travel from plane to plane?

Not really important until your players travel there.

Having decided it's time to build a cosmos, I advocate beginning with some basic questions:

How are planes differentiated? What makes them unique?

Planes can conform to alignments, which is standard D&D.

Planes can be the realm of a god, so that each god controls her own realm.

Each plane can be home to a pantheon which shares it.

There can be a hybrid of these or s6mething else entirely. The idea is to answer the question, "What is a plane."

The next question is, how easy or difficult is travel between planes for mortals? For servitor or minor immortals? For gods? Is there an obstacle outsiders must overcome to visit other planes? What protects the material plane, (if anything,) from being overrun by outsiders?

Once you have the mechanics worked out you can work on the fluff. If you opt to do so you can begin with the fluff, but I find it easier the way I have it described.

So, I begin with a pantheon per plane. Some deithes hold territory in the plane while others do not. Alignmemt has nothing to do with an afterlife, but one's devotion may allow a better eat at the table. Each god and her most devoted followers exist in that portion of the plane that she finds most pleasing and travel is limited only by internal agreement. Those who control a segment of the plane shape it to suit their preferences.

Travel to other planes is difficult because each plane is a bubble on the astral sea. Penetrating the bubble without bursting it is easier for beings of lesser power.

Each outer plane is adjacent to the material plane, so deities can send messengers across and can hear and answer prayers. If a deity wishes to enter the world she must do so through an avatar, and only a tiny portion of a deity's being can fit.

Different areas of each world follow different pantheons, but while where one is born has a strong influence on which pantheon one follows, where one dies is not relevant.

Spells regarding death work as described, and the pantheon's death deity dictates what happens to each soul according to the laws of the pantheon.

The shape of the cosmos is that of foam floating downstream on a river with no banks.

[jjordan]

How much pre-existing lore do you want to be able to incorporate? If you go full custom you have to rebuild everything. If you go with something that allows you to build in pre-existing material then you can selectively pick and choose as needed. So my advice is to leave yourself a lot of room.

[sktarq]

My advice<
iterate. and iterate more on the areas that your players interact with.

the problem with coming up with the cosmology later is that it binds you in consequence wise. If you want to add something that will make an interesting effect to the game world you have to ask what would the consequence of this change be (such as regular bound genies, planes that phase in like Khorviare, etc), what would be the second order consequences of those effects (different power groups, laws and customs related to various things, how worship, the dead/undead etc are dealt with). Now the question will come up is why have your players not seen these consequences in their play so far is you don't have it set up early.

Sure you can get around it but it does limit your choices.

So there are two real aspects here. First is just get the general idea of what you want the cosmology to be. Figure out the big consequences and see how they are likely to interact with the low level adventures you have sketched out. Maybe it is a good reason to adjust those adventures to minimize interact early on if you don't want to deal with it for now. (for example have them move away from major power centres that interact with planar stuff or (for example) just make the local duke very anti-undead and while the players may not be sure how the rest of the world treats the undead they know to expect a major shift later and there to be some sort of variable views on it...or as another example the existence of the Ring of Sybaris is one of those background things in Khorvaire. but if you suddenly add it and replace spellbooks with Khorvaire dragonshards when your players hit 4th level it will be jarring and you want to avoid it...for another example knowing that there is a way to pull massive amounts of elemental matter into the world with a gate like system and that has led to huge foundries of iron ore pulled from the EPoEarth and smelted by a second portal linked to the EPoFire and major irrigation projects are sourced with EPoWater Portals....may make the economics and challenges of how the little farming or mining town the players start in be rather different) When you figure out what parts of those consequences the players will deal with sketch those out a bit deeper.
Then you can go back and kind of go over it again make some tweeks, figure out what other consequences might be, how other groups could see or exploit the world as you have built it. Leave some quick notes for yourself. These things can be dropped as hints and background during your game that will give your world the feel of being fully developed without it being so yet. See what part parts touch your players path and work them up enough to use them at the table (this is mostly a matter of how much you ad lib etc).
IF you need to do it a third time...fine but it can probably wait for a while until a couple of sessions have helped you build a better feel of the place by running it a couple times. You can see what hints have caught your players interests and may have ideas of where to build out from there and you'll have enough groundwork laid that the players can follow on.

Second major bit of advice, work backwards.
Figure out what story effects you want, and work backwards.
How do you want various planar monsters it be seen in the world, do you like using them?
Do you like running extraplanar adventures and at what power level, and what kind of planes etc. . . this should give you ideas on how easy it is to hop planes and possibly how to do it.
Same with dealing with gods and the like? do your want your players to be able to hit them in the teeth? become one? or be left in mysterious awe?
are there any themes you want to see at your table? How can a cosmological change support this?

Basically it is the opposite of the above. think up consequences you like and figure out how to support them

[JoeJ]

The easiest way to create a cosmology is to take one from real world mythology and just change the names and a few details. You'll probably find elements that seem bizarre or contradictory: don't try to resolve them. If the sky god was born on a nearby island, but also is older than the world, that's fine. As strange as it may sound, that sort of weirdness adds verisimilitude.

Or possibly don't change the names, and you'll save yourself the trouble of having to explain to your players who the various deities are. Just tell them that everybody worships the Norse gods, or the Greek gods, or whatever, and leave it at that.