Ok, so this is kind of a meta worldbuilding question, but it doesn't really fit meta. It is more about worldbuilding itself. I have been browsing the site for a while, and I have gathered that things that are more powerful than omnipotence, multiple omnipotents, and unbeatable weapons are impossible, even in any fictional world anyone could make. What I don't get is that sphennings said that in worldbuilding, one doesn't have to follow any rules and nothing is impossible or cannot be done. But Tortiliena has said that there can be no unbeatable fictional character. So how can everything be possible in fiction, but an unbeatable character be impossible in fiction? Can someone please explain this?
As an author, you are free to have an unbeatable character. David Oglemann. His punches topple mountains. He eats killer bees for breakfast. He eats them through his eyelids. He doesn't need to eat of course. He does it for fun. And he is immune to all harm. Both physical and emotional harm.
But in the following chapter you are also free to introduce another character. Ovid Daglemann, who taps David on the shoulder and reduces him to a fine paste.
"But David said he was unbeatable!" cries the reader.
Ovid shrugs, "I guess he was lying. . . . "
"But the third-person narrator said David cannot be harmed!" laments the reader.
Ovid rolls his eyes. "I guess it was third-person limited."
As an author you are free to put anything in your story. You are free to have flying pink elephants. You are free to have potatoes look like bananas and vice versa. You are free to set the whole plot on a spinning ball of rock filled with molten goo, hurtling through space. You are free to snork meeble-feeble OMOOOO 56gft***;.
See that last bit? It doesn't mean anything. That's the problem. You cannot include that bit in your world because it does not describe anything. So you cannot put the thing it describes in your world. The same applies with a square circle. Or two omnipotent beings. It just doesn't mean anything.
Of course in fiction you're free to do whatever you want. The only limit is what you're capable of describing.
But if people are coming here to ask questions about worldbuilding, they're probably asking how to do something in a way that's plausible, or self-consistent, or at least entertaining, and those things do place limits on what you can do.
Sure, you could have an omnipotent, unbeatable character if you want, but you'll struggle to make an interesting story out of that.
The Language of Rules
Here on Worldbuilding.SE we spend a lot of time talking about rules. The rules of the Stack. The rules of your world. Rules, rules, rules, rules, rules. Some of those rules define the limits of user behavior. Some define what kinds of questions we're willing to answer. But there's one that's unwritten. And it's amazing that we need to even mention it, but it's the crux of your problem.
All rules are expressed in language. Call it Mathematics or English or anything else you like, if you can't express the rule using language, then you also don't have a rule and our one-and-only goal on this Stack is to help you define your world's rules.
And that means we have a problem when a user posts a question on this Stack and ignores the definitions of words.
Omnipotent: (of a deity) having unlimited power; able to do anything. (Oxford English Dictionary)
You, as a world builder, may want to explore the idea that there is something more powerful than an omnipotent being, but in doing so you've violated the definition of the word and by asking that question here you've placed us in the unenviable position of having to figure out what on Earth you're talking about.
From that moment forward, you're telling a story (off-topic), not building a world (on-topic), because without the basic definitions of words we all share and need to share to communicate meaningfully there's no way to help you do anything. And you can't simply say, "I'm not changing the definition, I'm exploring something more than the definition encompasses" because to a group of principally science nerds, that makes even less sense.
Mathematicians understand the concept of infinity very well. What is infinity + 1? Infinity. What is infinity - 1? Infinity. Is there something greater than infinity? Yep, infinity. If you divide infinity into one, what do you get? Zero (an infinitely small number). If you divide infinity into infinity, what do you get? We don't know! Because there's no way to meaningfully judge which of the two measurements is larger than the other.
And that's the problem you're having with asking us to let you ask questions about characters that are stronger than all-powerful characters. Because there's no way to meaningfully judge which of the two characters is stronger. You're not asking questions about the rules of your world, you're playing games with language and asking us to play along.
Questions must be specific as well as answerable. If you are looking for discussion, brainstorming, or an overall process rather than specific questions and answers, the Worldbuilding Stack Exchange might not be a good place for your question. (Help Center)
Questions that play games with language are neither specific nor answerable and we, the community, judged that we will not play that game a long time ago. We will not answer questions about how to overcome godlike creatures because in the world of "specific and answerable" the one and only way to overcome a godlike character is for the character to not be godlike. The character must have defined, specific, qualitative weaknesses that can be exploited and overcome. Without them, you're asking questions that would make more sense on philosophy.SE (and would likely get closed there because they don't play games, either).
If you want help developing the differences in strengths and capabilities between two species, races, or (maybe...) characters (we don't like focusing on individual characters) or anything else (like weapons), then you need to give up using words that have absolute and well-defined meanings in ways other than they were intended. We're not interested in helping you figure out a definition for infinity + 1 that is anything other than infinity because that violates a rule on this Stack: questions must be specific and answerable.
So, while it's true that you, the author of your world, are welcome to do "anything" at all in your world. We, the community of Worldbuilding.SE, cannot and will not embrace "anything." We have rules. And within the context of those rules, we will help you define and consistently use rules for your world.
And while it's not written in the Help Center, we nevertheless expect you to appropriately use the definitions of words and to avoid undefinable paradoxes. Can God create a rock that He cannot lift? That may be an interesting philosophical question.
But it's an inappropriate worldbuilding question because infinity divided into infinity is undefined.
What you need to remember is that all writers want to write good stories. When someone says 'You can't put such and such in a story', what they usually mean is 'Good stories do not have such and such, and since you are trying to write a good story you must not include it'. You can write anything, but your desire to write a good stories limits what you can do.
Of course, nobody has ever objectively proven what makes a story good, and there are even stories where the whole point is that they are as badly written as possible [This Gun in my Right Hand is Loaded is a great example]. For this reason, it is generally better to statements like 'You can't do this' or 'You have to do that' as guidelines to follow rather than laws that must always be obeyed.
"Impossible" is too strong a word to be applied to fiction. Things are impossible in the real world because they violate causality or other laws of physics. In fiction, the only limit is the readers' imagination.
Note that I didn't say the writer's imagination. Technically, everything is limited to the writer's imagination, but in reality, fiction is a form of communication, and communication can't exist without both a speaker and a listener. When you communicate, you create a set of rules that both sides are expected to follow. If you don't follow those rules, then you can pay a hefty price.
The easiest example is betrayal of suspension of disbelief. If you set rules early in your fiction, and then don't follow those rules later, your audience will be left dissatisfied. If, for instance, you meticulously adhere to hard science, and then solve a problem with telekinesis, then your audience will cry "deus ex machina!" And they should, too. Similarly, if you place something in the Wizardly World, and then it turns out that the bad guys are space aliens, it throws your readers off.
You absolutely must write in limitations for your characters, otherwise there is no conflict. If everything is easy, then nothing is interesting. This is the Mary Sue rule. It's why Superman stories are so hard to write. His limitation is that he runs with a perpetual hard handicap of not letting anyone die.
Another thing that a character can't do is nothing. A completely passive character would be a severe challenge, and I've never seen it pulled off. It makes character growth impossible.