A while ago, I did a rant on putting characters through hell. While I’m a big fan of that, I’m also a big fan of not getting melodramatic, and making it seem as though the hellish events actually fit into the story. So…

1) If the extreme event has to happen early in the story, make sure to show the roots.

Starting a fantasy off with a character getting raped, abused, nearly killed (probably not all killed, unless the story proper is going to be about a revenant), whipped with chains, in mourning for a dead love, and so on, has a few advantages. It may get readers into the story. It may be easy to write (although not always easy to read; see point 5). It may seem as though it will introduce a lot of dramatic tension and conflict that the characters have to resolve quickly, before it explodes.
But there are problems with it, too. Some, which I’ve ranted about before, are that it can make the character seem like a victim instead of a hero—particularly in the case of rape or abuse—that the author crosses what I call the “sadist line” and simply puts the audience off, or that it sets up a tenor that clashes like hell with the rest of the book. If the book itself is going to be a light and fluffy fantasy, starting the story with a desperate battle will seem more than a little odd. And if the character is badly wounded in the start, only to be almost instantly cured by a magical healer, then it can make it seem as if no characters will ever be in danger. There goes the serious high fantasy, along with the suspense, down the drain.
It can have another disadvantage, too. Quite simply, we don’t know anything about these people yet, we have no reason to care, and the author is trying to make us care by a cheap method. “Of course you have to care about someone about to suffer disaster!” is the assumption here.
Um, well, no. At the beginning of a book is when I’m most likely to remember that this fantasy is fiction, when my suspense of disbelief isn’t up yet and I’m not involved in caring about the people in front of me. And if I sense that the author is emotionally manipulating me into caring for the person, or just doesn’t have any other idea to start off with than a burst of melodrama, then I’m more likely to back away.
All of that was necessary before I could get to the main point, because I wanted to cut the heads off both common counterarguments—“That’s why it’s a good idea to start your fantasy with a potted mythic history/backstory of the character” and “Melodramatic beginnings are good”—before they could arise. Balance, as almost always, is best, and in this case, the story might demand an extreme event right near the beginning.
Fine, then. Show us the roots of that event. If it’s the second or third book in a series, you’ll have less work to do, but if you’re handling this in the first one, you’re in a dangerous position. You should give, as quickly as possible, some hints about the relationship between the protagonist and the character who’s trying to kill/rape/abuse/do something else hellish to him, and why this is happening. If it takes the character completely by surprise, then give hints about what kind of person he is before the extreme event takes hold. I feel a lot better when I know why something is happening, or, failing that, who it’s happening to, than when I open a fantasy book and see the author flinging blood like there’s no tomorrow.

2) If it can happen later in the story, build up to it.

hate it when the author builds up the relationship between two characters to be completely trustworthy and loving, with no hint that something is wrong, and then, later in the story, suddenly claims that Character A has really been in love with Character B’s husband for years and years, and thus is now going to tear Character B to pieces with flaying knives. It’s supposed to be suspenseful. It is not. It ‘s cheating. It’s a bolt from the blue that there’s absolutely no chance to prepare for—not with foreshadowing; not with buried clues that Character B thought meant something else at the time; not with Character B escaping some other minor attempt to kill her earlier in the story and attributing it somewhere else.
The author thinks she’s being surprising. I just assume she’s being an idiot.
To make an extreme event work later in the story is, I think, easier than planting one right at the beginning, but at the same time, you can’t just go “plunk!” and think the audience will accept anything. There have to be clues. There have to be, if it’s going to be a revelation rather than just a final, hellish confrontation between the heroine and her enemy, events that can be plausibly explained to mean something else. There have to be moments when the story could have turned in a similar way, and got repelled by time or the heroine’s actions or the Big Purple Aliens arriving or something else. There has to be a sense that this is coming.
No, it doesn’t mean that you have to include chapters from the villain’s viewpoint in which she gloats about how her evil plan is working. But do give the buildup at least some room. If the event comes as the final cataclysm at the end of a long chain, the main focus should be the buildup, as things get further and further out of control until they tumble into the abyss.

3) Use the characters’ surprise to the story’s advantage.

So Karabian is walking along, and someone drops behind him. He turns, and sees the first enemy climbing over the wall. He shouts, and grabs for his sword.
Often, at this point, one of two things happens:
a) Karabian’s shout wakes everyone up, and everyone helps him fight back the invading horde.
b) The enemy knocks Karabian on the head with his sword, and when Karabian wakes up, he’s in a jail cell and doesn’t know anything until people come to gloat at him and recall the battle in flashback.
Both don’t help your story that much. A) makes the victory seem too easy, and, at its worst, useless; I’ve read several fantasy books that seemed to include battles just because the author thought there had to be action. B) skips a scene that the author could have made interesting, sacrificing the advantages that action does offer, and brings in yet another infodump conversation.
Instead, if you surprise your characters, and you want the surprise to be an extreme event, work the surprise into the fabric of the plot. For situation a), right after Karabian wakes everyone up, he could learn that his enemies use a fire that burns flesh but nothing else. The death toll’s going to be much higher when the soldiers come rushing out than it would have been if they had hidden behind walls and doors. For b), Karabian could learn news that would be much worse than the usual “We took your castle and killed some dispensable sidekicks with names and at least one innocent iconic child, but of course some people got away, including your best friends, ha-ha!” speech that most heroes get. Perhaps he really is the last survivor. Perhaps the confinement itself is deadly in some way. Perhaps the villain tells him a lie that he believes and does something stupid because of.
All of these would be much better extreme events than a) or b). Neither of those really qualify, in fact, because a) is not hellish, and b) doesn’t let the hero, or the readers, see the event as it happens. (See point 6).

4) Don’t plant too many reassurances, or too immediate.

I’m currently reading the “A Series of Unfortunate Events” books, which deal with bad things happening, constantly, to the Baudelaire orphans. The author uses a writing style that would normally both bore and annoy me to death, because he often says something along the lines of “But of course they would see them again,” reassuring his readers that those horribly endangered characters won’t die after all. However, he counters this with “reassurances” that go like, “I am sorry to tell you that, in less than a week’s time, [Character X] would be dead.” He then shows the horrible things happening instead of doing fake foreshadowing, and the good things coming true in less reassuring ways than it sounded as if they would at first. The style works because of the balance. Too much one way or the other would kill it, and work against the dominant, extremely black humor tone of the series.
In most fantasy books where the author goes out of the way to erect a lighthouse guiding his readers to a comforting conclusion, there isn’t that balance. There’s just the author having one character dream prophetically about another character returning to life, or having someone say a variation of, “I have a feeling that it’s going to be all right.” And of course it is. (Sloppy writing. Very sloppy writing, right up there with “somehow.” And yes, people in real life get hunches like that, too. But ours don’t always pan out. The ones in fantasies do, and the author doesn’t even have to explain where the characters got them. Fucking lazy-ass shortcut).
I hate, hate, hate these. You have just slain the suspense. And it makes even less sense to surround an extreme event with them, because in this case, you want the reader to feel dread and fear and terror. Dissipating that too soon, and through such gadfly methods, works against your own story.

5) Try using simple language instead of purple.

See the action scene. See Limyaael reading the action scene. See the action scene crowded with words like “stygian” and “ambidextrous” and “amethyst.” See Limyaael get lost trying to read around the adjectives and find out what exactly is the hell happening.
I understand the impulse to clutter the page with adjectives and adverbs during a final confrontation, a betrayal, a battle, or whatever. It’s supposed to be grand, so the writer wants it to be grand for the reader, too. And because we naturally assume that a big vocabulary and lots of description (especially lots of description of colors) is grand, we go for that.
But it often hurts an action scene. If your audience doesn’t understand what you’re saying, they lose the flow of the writing just as you want them to get most involved. And the extra words clutter the scene, which is death to the speed that a great many confrontations need.
Try stripping down your vocabulary to nouns and verbs on this one, and the simplest adjectives and adverbs, like “dark” and “two-handed” and “purple.” If you try it and it doesn’t work, well, fine. But I’ve read far too many action scenes where authors forgot about this, and tried to treat the battle on the top of an icy cliff the same way as a fancy dress ball. I am not particularly interested in what the character wears while he’s fighting for his life, or how pretty the snow looks when it’s spraying up from the impact of his foot, or whether the blade of the sword against his neck is made from steel that comes from Damascus or not. Concentrate on what’s important first, and bring in the ornamentation only as you need it.

6) Take a viewpoint as close to the event as possible.

This goes along with points 3 and 4. Some authors seem to feel that their readers cannot possibly handle the hero almost dying, or being betrayed, or having to kill his best friend. Instead, they set up the scene, then sweep away and do the “omniscient voice in the sky” business: “If a crow were hovering over the mountain at that moment, it might have seen…” Or they knock the hero out and have the event recounted in flashback later. Or they suddenly jump to a minor character’s eyes and have him watch the duel, instead of putting the reader in the head of someone actually involved, and who he’s probably followed for most of the story.
I think this is pretty damn stupid if the event is supposed to be hellish. Being told, second-hand, that the event is or was hellish is always going to be less inspiring than experiencing it firsthand. Switching viewpoints in the middle of the scene is clunky unless you practice. Flashbacks are a tool to use when you don’t have another choice, not just a handy alternative to the main narrative. Minor characters are, or should be, more than eyes on the action. Why in the gods’ names would you abandon your main, suffering character when you’ve gone to all the trouble to make him both main and suffering?
Trust your readers. They’re often big boys and girls. They can take full immersion. And so can you, I bet. If you go through it with your suffering character, and feel a close enough connection to suffer yourself, then you’re going to write it better.
Once you’ve earned your extreme event, you have to have the courage to go through with it.
I will never understand authors who go to all this trouble to build up hell and then shy from it when the moment approaches. I can understand the impulse to do so. I can’t understand actually abandoning the track that you’ve willingly followed.