I am working on my first homebrew setting using a spelljammer sort of cosmology, and I have run into some confusion. So far, my understanding is that gods in D&D only exist where they have followers. How, then, does a god create a world from nothing? If no world = no worshipers = no god to create the world?
He was credited as the creator of the cosmos, and even existed beyond concepts such as alignment and divine rank.
Ao had the power to create gods out from thin air.
...unlike the gods under him, Ao did not need the worship of mortals and did not desire it either, whereas those "normal" gods who did not receive the worship of mortals could die from lack of it.
So in answer to your question, there are Overgods who do not need worshipers.
Gods don't necessarily need worshippers
The idea that the power of a god is derived from the belief placed in the god by its worshippers is a common one in fantasy, but it is not universally true in the genre generally or in D&D specifically.
The first chapter of the Dungeon Master's Guide, A World of Your Own, details the Core Assumptions of the game and has the section Gods of Your World, about the different kinds of pantheon and belief system you can use. It is a default assumption of D&D that gods exist and supervise the world in some capacity, but neither of these sections ever suggests that the power of a deity is in any way constrained by its number of mortal followers or that a given deity needs to have followers at all - this idea is almost completely absent!
The only place it comes up here is in the Forces and Philosophies section of this chapter, which describes allowing characters who might normally be divinely empowered to instead draw their abilities from personal belief and conviction in a philosophy or ideal, but notes that:
The power of a philosophy stems from the belief that mortals invest in it. A philosophy that only one person believes in isn’t strong enough to bestow magical power on that person.
But this doesn't generally apply to the gods as described earlier, and in context I would interpret this as being necessary only because there isn't an actual god to provide the power instead.
Of course, if you have a world where the power of gods is associated with the belief invested in them by mortals - say, the Forgotten Realms - this question is valid. So what about the Realms, then?
Gods didn't always need worshippers
It is true that in the Forgotten Realms, the power of a deity is explicitly tied to their popularity amongst mortals. A god with a large portfolio and a huge flock of worshippers is more powerful than a god who has only a small portfolio and receives few prayers - remembering that FR society is generally polytheistic, and a deity who does not necessarily have a large number of dedicated followers may still receive many prayers from those dealing with their domain and derive strength this way.
However, the world of Aber-Toril was supposedly created by Selune and Shar, two goddesses that formed out of primordial nothingness and created the whole world of the Forgotten Realms before any mortals existed. How did they do this if the power of a god in the Realms depends on their mortal followers?
The simple answer is that, at the time, it didn't. Before the Time of Troubles, a god's power was derived directly from their portfolio and their mortal worshippers were irrelevant. One of the consequences of the crisis was that the overgod Ao, angered by the deities' disregard for their followers, decreed differently.
The close of the Avatar Crisis brought a change to the way the deities of Faerun relate to their followers. By Ao’s decree, a deity’s power is in part derived from the number and fervor of his worshipers, and so deities can no longer afford to ignore their faithful. (3e Forgotten Realms Campaign Setting, p.264-5)
Of course, in FR's case this requires the existence of an even greater divine entity, the overgod Ao, who created the universe all these gods exist in and has the power to simply decree things like that. But your setting could take a similar approach, with the original prime mover being an inherently powerful but aloof deity of some kind, and the more active gods of the world being those created or empowered by mortal worship instead.
What you should always remember as a DM is that nothing about the "official" D&D lore is a hard rule consistent over every possible setting. Things like "gods need followers" or "chromatic dragons are evil" are assumptions that the game makes about your world by default and that are true in the Forgotten Realms or other official settings. In general, you should follow these conventions if you don't have a particular reason not to, since it makes everything a bit more consistent and it makes sources like the Monster Manual easier to use - and it saves you the DM a lot of work.
But if one of your core ideas about your world contradicts one of these assumptions, you should definitely go through with it. If a rule or a piece of lore is in your way, you shouldnt be afraid to change it in order to make your game more fun. So if you like the idea of gods existing regardless of mortals to revere them, do it. It only means you may have to change a few more aspects of the default D&D to suit your idea.
This answer is about the official Spelljammer/Planescape lore, from which you can base any homebrew. You are, of course, free to change anything you like, but I think for this question you don’t need to be told that.
The first planes to exist were the Inner Planes, the elemental planes and planes of energy. The Material Plane formed from the mixture of the elements and energies, and then life appeared upon it. The beliefs of living things then created the Outer Planes, and thus most deities post-date life. (Even though a large amount of the variety of life on the Material Plane is due to divine intervention, there must have been some life before them. Aboleths and le shay, certainly,1 though there must have been more than that, probably something or somethings humanoid.)
The Material Plane is divided into Crystal Spheres, each of which generally contains a solar system. Realmspace for the Forgotten Realms, Greyspace for Greyhawk, etc. Between the Crystal Spheres is the phlogiston, where spelljammers travel. And each Crystal Sphere (presumably) has an overdeity, e.g. Ao for the Forgotten Realms.
It’s not really made clear where the Crystal Spheres and there attendant overdeities come from, or how any of this happens. As far as anyone who’s talking knows, both were just always there, at least for as long as the Material Plane has existed. Maybe an aboleth or le shay could shed light on the matter.
But the long and short of it is that Crystal Spheres and overdeities come part and parcel with one another. Overdeities are extremely enigmatic and aloof, so very little is known about them, and it’s not really clear what—if any—limitations they have with respect to their Crystal Spheres. I don’t believe we even know if overdeities get to create the worlds within the Spheres, or if those come with. Ultimately, it doesn’t have to be a thing you really decide, because there’s almost no way anyone is ever going to find out those answers. You’d probably have to ask an overdeity for most of them, and as a rule those don’t answer questions.
Regular deities, rather than overdeities, definitely don’t have much of anything to do with it, though. Regular deities can shape things to an extent, and can definitely create life forms of great variety (e.g. Corellon Larethian is known to have created the elves, Moradin the dwarves, Gruumsh the orcs, so on and so forth), though they aren’t the origins of everything (e.g. humans don’t have a creator deity).
Anyway, overdeities don’t require worship, it’s only regular deities that do. Really, in most ways, overdeities aren’t like other deities at all. They are linked solely to one Crystal Sphere, they don’t have a divine realm somewhere, you cannot meet them incarnate, and their power within their Crystal Sphere is, if not absolute, at least well beyond anything the deities who operate there can question. When Ao makes a rule that the deities of the Realms don’t like—and the Realms have a lot of onerous rules for deities—they can’t do anything about it at all, they just have to figure out how to follow the rule.
So it’s conceivable that overdeities existed prior to believers, it would just be yet another way in which overdeities aren’t really deities at all, but their own special breed above deities.
- Aboleths claim to be—and almost certainly are—the first sentient beings in the multiverse. The gods are, to them, upstarts. Le shay, on the other hand, predate the multiverse, somehow having survived from some previous multiverse to exist in this one. So they’d be the ones to talk to. I’d probably start with the le shay, if I had my choice....
A deity is more "present" where followers are found, but followers aren't related to their mere existence. For example, "Overdeities" are discussed in Faiths and Pantheons and those aren't even known by mortals.
If you can comment with the source you're referencing here, we may be able to work through whatever caused the confusion.
Your question is one of theology and entertains multiple mutually-exclusive-yet-coexisting answers from various D&D products. Given that each crystal sphere has their own creation myths and pantheons, we do better to not rely on campaign-specific answers, but try to look for information from meta-settings: Spelljammer and Planescape.
The one sourcebook on religions and deities in the Planescape campaign setting is On Hallowed Ground. This book makes it clear that everything on the Outer Planes is sustained by belief and that deities make use of this fact. Yet it also does not shy away from the following open-ended question/speculation (page 37):
Here's the chant ... : The Inner Planes, seat of the elements and building blocks of nature, appeared first. The Ethereal Plane came second, followed by the Prime Material Plane, where the elements combined and formed mortals. Mortals created knowledge, and knowledge formed the Astral Plane, the bridge to belief. And with the development of belief came the Outer Planes. ... since the theory implies that primes existed before the Outer Planes ... the idea isn't exactly welcomed on the Great Ring.
A similar open-ended theological speculation is presented in the Concordance of Arcane Space sourcebook of the Spelljammer boxed set (Creation of the Universe sidebar starting on page 42). The phlogiston that fills the "volume" beyond the crystal spheres and the flow is beyond the reach of the deities; priests on spelljamming vessels cannot loose contact with their gods. Six different hypotheses are put forward, one of which speculates that there are gods-of-gods which created the universe, another speculates that the shells of the crystal spheres are naturally occuring and that the deities simply occupied these spheres and lied to mortals about creating them. [You can read a summary on the Spelljammer fandom wiki.]
The answer to your question is not well-established in the lore. It is clear that belief is essential for most deities, yet it is left to each gaming table to decide the correct answer for others (or to decide whether a correct answer exists at all).
The Gods who created the universe need prayers because they created the universe
It sound like a bit of a tautology, but it is not intended that way.
Sure the gods are powerful. They can raise the dead, conjure up a +5 sword with a wave of their hand, and even create servitors (incredibly powerful and sentient beings).
But that's peanuts compared to a universe. Even compared to a single solar system (which most crystal spheres seem to be).
The act of creating the world. Its matter and magic. Its races. The very laws of physics and metaphysics. That took a lot out of the gods. And now they need to recharge. And that recharging mechanism is the prayer of their creations.
Of course, the greedy little things keep making demands in order to keep praying. Which slows down the recharging process. Every spell granted is a bit of power that the god could have used to regain her former stature. But if you don't give the clerics their spells, they and the people they minister don't worship as often or faithfully.
Not to mention rather than just sharing equally, or based on who contributed what, some of the gods are trying to snatch more than their share. Converting followers, or having their followers wipe out others.
tl;dr: They used to be Gods, but creating the universe reduced them to gods, so they need prayer to survive and to regain their former strength.