How do I reduce the necessary details in my story?

Many world-building details about the story's setting should not be crammed all at once for readers at the beginning, as most people won't have the patience to read through it all.

The information readers need to know should be conveyed naturally through the storyline, allowing readers to understand gradually. From my perspective, most of what you mentioned in your article can be accomplished this way.

If you absolutely must explain so much in the first chapter, otherwise the second chapter becomes unreadable, then you might need to adjust the entire structure and sequence of your story.

If the only way for a story to make sense is if there’s an exposition dump at the start, you’re heading into the plot too fast. Try and start off slower and introduce these ideas organically while getting the audience familiar with the status quo and protagonist.

Well, someone gave a simple advice: Read what you wrote, and search your feelings whether some parts are not to your liking, and you would later remove it. Authors are readers too.

Don't let the details be an entire page or two.

That's a lot of information, indeed. And I get your frustration. The earlier reply was right: don't info-dump everything in one go.

Since the world-building itself is complicated (it takes time for me to digest your whole story), you have to separate it into bite-sized information. If possible, use scenes, dialogues, whispers/gossip to deliver the information.

Here's how you can include major plots without info-dumping:
1. Create a scene where the Dark King is claiming a territory. (how the dark magic works)
2. Create a scene of the MC venturing into a barren/destroyed land. (what happens if light magic is used to fight the dark magic)
3. Show fear of Arcanum through NPCs, where they talk amongst themselves. (this matters later anyway, so no need to go deep dive into it right away)
4. Dark zone's isolation should also be shown through NPCs, perhaps nosy commoners or someone claiming to have a soldier relative and gossip about the latest news.

Don't do the above things all in chapter one or in several chapters in a row. Incorporate information as scenes or dialogues for easier digestion.

I suggest opening the chapter with MC 1's magic poisoning. Or if this is still a secret, you can open the chapter with the nature of prophecy stuff. How MC 1 was prophesized to go somewhere, but they refused to follow, and what happened to them as a result. Like your examples—turn them into a scene for the MC.

Readers want to see your MC—I want to see your MC. If this is still too difficult to achieve, perhaps you have to restructure your story. Maybe make the MC unaware of themselves being the chosen one, so he'll learn about it with the readers.

just don't write it.... leave some holes. therefore you will have room to expand on those holes when you need it.

I mean, you don't need to explain all of this in one go. You can break it up through chapters/episodes. A block of text talking about the basics and all of that is just not preferred.

Alright. You have a lot going on in your story - maybe too much. However, based on your descriptions, I don't think everything really NEEDS to be told at first. Further, while you gave a lot of worldbuilding information, you didn't tell me much about thse lead/chosen one people. What are their personalities? How are they interesting for a reader to engage with?

My advice is to focus on this second real but unknown chosen one (if you must have chosen ones at all) as your protagonist, someone who doesn't know he's the chosen one but does want desperately to help the displaced people in his land. Maybe he lives alone because his family was killed by the dark magic. Maybe he has a secret hideout where he experiments on some dark magic-infused objects, trying to purify them. Maybe he spends his days working in a menial job which imparts skills that will be relevant to purification later (salt maker? thief? candle maker? gardener?). His most important character attributes could be his compassion for others and his persistence to help people, and his personality could be relentlessly cheerful and optimistic to inform the dichotomy of lively light vs cold death.

Then the ruined former chosen one with magic poisoning can be this somewhat older, famous "chosen one" that the population trusts and whose magic poisoning is a state secret. You can now work all of this into later chapters as our hero navigates the world.

Just show that the world is this way, and don't explain it! It'll look cool and be fun. You can explain it later.

The other elements - how prophecy works, what the Arcanum is, how dark corruption works, that there even is a demon king, and so forth - can also all be introduced later, one at a time, as our protagonist learns about the world outside of his humble refugee camp.

With the first chapter, you need to establish ONLY these things:
1. Who is the lead character, and what are they like?
2. Why do we care?
3. What is the main draw of the story?

The setup for anything else shouldn't be more than a throwaway line or an image here and there at this point. I bet you can build a good opening chapter with no prologue for this story, if the lead is solid enough.

If you are worried there is too much necessary details, maybe cut back on what is needed for the story.

I would also recommend reading books in the same genre and see how other writers handle it. I feel a lot of them start with a naive hero who befriends someone who helps explain things to them through out the story.

As others have said, you don't need to reduce the detail of your story just rework it. Take a look at what your story needs to know right now in act 1 and explain the bare minimum details of what needs to be known for the upcoming chapter.Through character dialogue, character non-verbal interaction and attitudes you can get a lot of info.

Sometimes an ATLA-style info dump at the start can work out, but it needs to be well written, easy to follow and most importantly: hella short. A short paragraph max with at most 1 jargon word (and that word has to be EASILY DEFINABLE within the paragraph), and the visuals if in comic form have to also reflect what is being said in the paragraph. Though it can seem simpler, it's actually a lot harder to make intriguing.

Ok so I'm going to give an example from my work because it's what I know and it's one of the things readers have praised so far.
My story starts with a short 4 page intro to the world (page 1 pictured here), it sets the scene for chapter 1 and gives people the absolute barebones info on what's going on so that the reader can understand the immediate stakes of what happens directly afterwards, i.e. there's a truck giving out water that the MC is going to miss if they don't run now to catch it. It's immediately relevant information so i made the compromise of shortening the intro to get to the action and work out a narration-style intro that shows off the world while the MC finishes work and goes home.

However, cloning is a really important part of my comic. The main character is a genetecist who specialises in cloning, many characters are clones themselves, a huge point of tension in the plot is how the clones are percieved and how they fit into the world. I have an 11 page document on different cloning methods that the MC is intimately familiar with, the history of cloning, the big innovators of the field, the fight for justice in history, the works. They are arguably more important than the setting itself.
The clones are not mentionned explicitly until near the end of chapter 1, 30 pages in. And even then, only a single line of my notes made it in: that the mc has trauma that is used as an excuse for their prejudice that is common enough to be somewhat brushed off, though not entirely socially acceptable.

None of this is stated directly, it's all in subtext. Not always subtle subtext but subtext nonetheless. Other details like how clones are made exactly, life expectancy, human genetic modification laws, clone usage in labor forces, etc... All are important aspects to understand the stakes of where I'm going with this story as well as multiple character arcs, but they will be introduced when they need to be, because dumping what is essentially a lecture on the subject at the start is going to push readers away. I didn't make the story less complicated, I made it more digestible. You can too.

Also important to know: I am not at the second act yet, and I'm 80 pages in. I've got another 5 chapters before i'm there, and I've already been able to build on the barebones explanations I gave in the intro. Just because the second act is coming and you need readers to know these things first, doesn't mean you have to stick it all in the first 10 pages and then once we've done our homework the action can start.

I know better than to actively engage with your questions, lest I tumble down a frustrating rabbit hole.

Instead, I'm linking a writing lecture series by Brandon Sanderson and you do the work yourself