: How do I introduce dark themes? My story involves a superhuman organization that aims to overthrow the main government, through any means necessary. This involves murder and some rather gruesome

The fourth chapter is too late. You need to do this within the first 10% of the story (by page count), anything more is too late.

If your MC does violent things for good reason, you need to give the reader the idea they are capable of this early on. At the beginning of a story, the reader is ready for anything -- magic, superhuman powers, aliens, gods, immortals, interstellar and intergalactic travel, shape-shifters, fantastic creatures and people like Wolverine. Anything! But their patience and acceptance fades by the 20% mark or so, if you haven't introduced something by then, it starts to look like Deus Ex Machina territory, like you are just making new stuff up out of the blue to move your plot along. You don't have to intro every fantastical element then, if you show magic, all kinds of future magic become a possibility for the reader. If you show one alien (or even mention them seriously), other aliens are fine, a whole culture of interacting aliens out there would be fine. One weak telepath allows far stronger telepaths, etc.

For violent things, you can be direct: Perhaps he kills a walk-on criminal, and has no regrets or qualms doing so.

Or you can be less direct, a man is harassing a woman, he steps in and pushes him off. The man pulls a knife, our MC doesn't flinch, they grapple and break the man's arm, then his nose with an elbow, the MC takes the knife and is about to kill the man before the woman cries "No don't!"

If you want to be completely indirect, you can at least show your MC in high-level martial arts training against rubber knife and paint gun-wielding opponents, actually delivering killing blows on dummies, as a kind of foreshadowing about his capabilities. As Galastel noted before me, that is a Chekov's Gun, you don't devote pages to this kind of training unless it is actually going to be used.

Likewise you need to demo the MC's commitment to good, that the government they want to overthrow is just as brutal. You need to show that the MC is on the side of good, meaning the MC engages in altruistic acts, the MC doesn't harm innocents for selfish gain (or at least doesn't engage in irreparable or irrecoverable harm; i.e. the MC is redeemable).

Likewise you need to show magic exists. There is no reason all these things could not be shown simultaneously in a single scene: A criminal uses magic to assault somebody and the MC altruistically intervenes, violently.

I am not saying all these rules cannot be broken, or have not been broken by best-selling authors. But breaking them risks getting rejected, and beginners should stick with the standard formulary: Introduce your fantastical elements early, and if your MC has any special powers or skills, characterize them early. While the reader is still open to anything.

The same goes for any dark side, indicate it early, but I would not show their dark side before their good side; first impressions are massively important here: Good first, and in your case, the dark side in response to darkness: The MC fights fire with fire.

You need to invent a new scene; perhaps on the boundary of Chapter 1 & 2, a way to end Chapter 1, start Chapter 2, or be Chapter 2. It can involve throw-away criminals that are gone forever after this scene, but you need to show it. Chapter 2 or 3 would likely work, but by Chapter 4, we are supposed to already have a good handle on the MC.

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Other than the already given answers, setting the stage really helps. I'm going to refer to the short story we studied in high school specifically on how to introduce a story.

To summarize, a doctor (MC/narrator) is visited by a friend, missing his two legs and one arm. The friend explains that he's addicted to eating his own flesh, and asks the doctor's help in cutting off and cooking his last arm. It's gruesome, but the story initially starts out as two old friends meeting each other again, and the story gets darker as it goes along. The slow descent into darkness is the main pull of the story.

One of the major suggestions that this story is going to take a dark turn happens in one of the first sentences:

It was a dark and rainy November night. I was reading a scientific paper on the spreading of gangrene in malnourished infants, when I heard a knock on the door. I got up and ...

The environment (dark rainy night) helps set the stage, but what stands out more is the topic of the paper the MC was reading.

The topic could have been anything. He could've been reading The Hungry Caterpillar and the story wouldn't have changed one bit. But because the author chose to talk about a dark and disgusting topic, it immediately points out to the reader that the author is not trying to write a happy story for children. This story, much like the scientific study being mention, is going to be dark and disgusting.

Setting the stage can be done using things that do not matter to the story at hand. Simply make them equally dark and unsettling, and you'll be able to suggest to the reader what the theme of the rest of the story will be. Some examples:

A character's backstory contains a suicide/death => the story is going to contain death or existential crises later on
Someone casually tells the story of a legend (often to children) => the story is going to feature this legend or a similar one.
News report on a serial killer => the story is going to feature some kind of crime/threat on the same level.

You could even make it more subtle by referencing fiction-in-fiction. For example, for the last example, you could also talk about the MC going to see a serial killer movie instead of having a serial killer in the (in-universe) reality.

What matters is that you (the author) chooses to talk about a theme that fits with what your story will touch on. It doesn't matter how you reference it (in-universe reality, in-universe fiction, side character backstory, main character motivation, ...).

For example, Harry Potter gradually got darker and darker as the series went on. And even just the first book didn't have darker themes until Hagrid mentioned Voldemort for the first time.

"for the first time" is sort of the point, Hagrid set the theme then and there. By warning Harry (and also the reader/viewer) well before Voldemort became an actual character (i.e. actively involved in the plot), Hagrid opened the door to understanding that this story was going to have a narrative villain.

It immediately suggests to the reader/viewer to consider what an evil wizard would be, what they would want, and how much Harry would be in danger. This almost immediately fixes the good vs evil theme that the HP plot follows all the way to the end.

Just like Hagrid mentioning Voldemort to Harry, you could have some character talk about the dark thing in your story, e.g. anarchist riots, superhuman feats, civil violence, ...

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Foreshadowing is your friend.

Your example of Harry Potter isn't quite right. Chapter One is titled The Boy Who Lived. Now that's a bit ominous. Magic is hinted at on page 1* and is outright on page 2. "You-Know-Who" is first mentioned on page 5. By page 11, when the name Voldemort is first mentioned, we already know he was a danger and now believed to be gone (but it's not certain). The chapter goes on to describe Harry's parents' deaths, that he survived, and that he was being handed over to his abusive aunt and uncle. By the end of that first chapter, we know there will be a story about good vs evil that involves the little infant who somehow survived the villain who murdered his parents.

Your book doesn't even have to be as direct as this middle-grade/young-adult series begins. Just have some strange stuff in the background. Newspaper headlines, whispers on the street, TV news reports. Make it clear that this is a world where XYZ can happen, even if it hasn't happened yet in your story.

Show us this world where magic, darkness, and death lurk around the corner. People live their lives normally in the meantime, but they know these things are possibilities. Or maybe they don't know...but they hear of strange happenings (things that happen to other people). What's important is that your reader knows they're possible. Then it won't come as a surprise.

* My pages are from the American trade paperback edition.

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Four chapters in, your readers should have an idea what they're in for. Not everything that's going to happen, but certainly a hint. Once you've hinted that there is darkness, you can skirt it, turn your back on it for a while, or plunge right into it as you see fit in different parts of your story. But it can't just show up out of nowhere more than a quarter of the way through.

Yo mention Harry Potter, so I will use that as an example. Voldemort is in fact mentioned right in the first chapter, by Dumbledore and McGonagall:

'What they're saying,' she pressed on, 'is that last night Voldemort turned up in Godric's Hollow. He went to find the Potters. The rumour is that Lily and James Potter are - are - that they're - dead.'
Dumbledore bowed his head. Professor McGonagall gasped.
'Lily and James ... I can't believe it... I didn't want to believe it ... Oh, Albus ...'
J.K. Rowling, Harry Potter and the Philosopher's Stone, chapter 1 - The Boy Who Lived

Death isn't going to be brushed over in this story.

'It's - it's true?' faltered Professor McGonagall. 'After all he's done ... all the people he's killed ...
ibid

That Voldemort was a serious monster. And there's no sense of giving him this much page-time if that's the last we're going to hear of him.

After that first mention, we spend quite some time without anything "dark" happening. Hagrid mentions Voldemort and gives us more information about him when he meets Harry - we get another hint that Voldemort is important. But overall, most of the first book is "fun in magic-land". Nonetheless, there in the first chapter we saw a gun on the wall. When the gun fires, it's not out of the blue.

Every time Voldemort's attempted return is thwarted, the gun jams. But a gun that doesn't fire is boring. Eventually, the gun has got to fire, Voldemort has got to come back. In the first chapter of the first book we got a promise of darkness. Sooner or later, the promise has got to be realised. And each time the promise is repeated, our anticipation grows. So when Voldemort finally returns, there's got to be enough darkness to satisfy the promise.

That's what you've got to provide in your first chapters - a gun on the wall. Whatever your dark themes are, hint at them in the first chapters, hint some more a while later. When you need the darkness to finally show its face, you've got the gun on the wall - all you need is to take it off and pull the trigger.

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