Another rant I wasn’t really looking forward to, because it runs almost opposite to my own inclinations. I prefer to write about stories of discovering knowledge, especially ones where the characters discover that the metaphysical/religious/mythological structure they took for granted has altered. “Forbidden” stories rely on a different appeal altogether, one I probably understand as a reader but not as a writer.

1) To tale or not to tale?

There are different kinds of forbidden, after all. There’s the kind where people just step into a tower and never come out again. There’s the kind where someone sets out with the stated goal of crossing the Forbidden Mountains and then is never seen again, but for all the people left behind know, they chickened out at the last moment and went somewhere else instead. (I want to see more fantasy books use that as a plotline, rather than the one where someone sets out to find a missing friend and learns that the friend did cross the Forbidden Mountains… ahem. Um. Yes. I mentioned I didn’t like this, didn’t I? But I must stop looking for ways to undermine it. Bad authors do that enough already). This is the sense of the forbidden based on the unknown. Anything could be waiting in the tower or across the mountains.
Then there’s the kind where there are numerous stories about what will happen to someone who draws the Sword of Dralcur when it isn’t the full moon, or of the various inventive ways that bodies get mangled when people venture into the werewolves’ forest. This is the kind where people know the consequences, and the consequences are nasty. The taboo or warning was set up in response to a justified fear and accounts that, however exaggerated they may have become, the original forbidders apparently had some reason to trust.
I think both can work. I do ask that authors stay consistent. If the characters have a conversation about the werewolves’ forest early in the story and are daring each other to go in because people no longer believe the nasty stories, I want to know what the hell is going on when they get to the edge of the woods and then say they have no idea what will happen when they venture in. You did earlier in the story, you idiots!
Basically, if no one ever come back and no bodies are ever found, how do people know what happens? That’s where you plausibly get into stories that may be concocted, and where the two senses of the forbidden can be blended. But if the basis for fear is totally inconsistent, I would expect someone to notice.

2) Stay true to the characters’ cultural and educational background.

It’s been ten years since I read them, but I still remember admiring the sense of taboo that Sean Russell used in his “Moontide and Magic Rise” duology, World Without End and Sea Without a Shore.The main characters had grown up steeped in a world where science, in the form of natural philosophy, had made significant strides—probably the equivalent of the 1700’s in Western Europe, although don’t quote me on that. (Why is it that I can remember, in detail, the Robert Jordan books I don’t want to remember, and not this?) Magic had once been part of the world, but had conclusively died. When the characters began encountering magic, and the taboos that guarded it, in the southern islands, they had no idea what to do. They were laughing at what they saw as the ridiculous nature of the natives’ ideas about forbidden things, but at the same time, they were terrified.
That was a much better picture of people reacting to ideas about the forbidden, condescension and all, then the frequent one I see in stock fantasy: everyone except the main character, who of course is just that smart, cowers, while he strides up and breaches the forbidden barrier and does all sorts of grand things. Without consequences, yet. (See points 4 and 5).
I don’t care how little you would personally believe in your characters’ ideas about magic and what actions, beliefs, or objects are forbidden. If you make it silly in the context of the story, you’re poisoning your own story. Why should a reader take it seriously when it’s obvious that the author is laughing up her sleeve? In a way, this is a twisted variant on message fantasy. I know what I’m in for when the author sets up an oppressive male-dominated society versus the small group of special witches who make magic from the moon and menstrual blood. I know who to cheer for and what the author wants me to think. Likewise, I know what the author wants me to think if everyone believes in the taboo but the protagonist, and the taboo is something like, “Never walk on the yellow sand during the day.”
If you don’t see this as a problem, go away, please. I can accept laughing at the forbidden in the context of parodic or satiric fantasy, because there everything gets laughed at. I feel my intelligence is insulted when the author creates a ridiculous society and then has no one but one person question its ridiculousness, and absolutely no consequence fall on him for doing so. Apart from anything else, it goes back to my old beef about the one moral or non-sexist or non-racist character in an immoral or sexist or racist society. When everyone around him received and believes in different messages, where the fuck did he get his ideas?

3) Trappings don’t do it.

Anyone out there still impressed by “spiky-looking runes in the snake-language of Ngarthak,” or “blood-soaked rites performed at the dark of the moon,” or “altars covered with runnels of something too dark to be water”? What about screaming skulls or axes that rise and chase people around on their own?
Yeah, me neither. (And, if you like, you can take that as a summary of why the sword-and-sorcery subgenre and I have an argument that’s not going to be resolved).
The trappings of the forbidden, when they’re just trappings picked up from a thousand other stories, will not cut it. I don’t care if people in your story talk in hushed voices about the Door That No One Can Open at the top of the Mountain That No One Can Climb in the Town of Dull-as-Fucksville. Put some goddamned heart into it. You didn’t invent these trappings, did you? No. So you can’t trot them out and expect everyone to be impressed by your shiny new ideas. You have to add a twist. You have to show why they work as trappings of the forbidden in your world.
Fantasy in general has a whole set of conventions that most readers accept and don’t question. I think this is a bad set of them, because—just as with mystical magic—a sense of the forbidden doesn’t depend on just the right bit of dialogue or just on how politically correct you are with the gender relationships. It’s an attitude. Many, many components of the story have to work together to support it. And when you take away the nice sturdy stone pillar that should support it to substitute a creaky old wooden column from pulp fiction instead, you’re doing it a disservice.

4) “That way lies madness/that way lies death.”

You have the forbidden [action/person/place/spell] in your story. It’s going to be breached. You already know that. Your characters are going to have to walk through the werewolves’ wood, or draw the Sword of Dralcur at the dark of the moon. It’s almost a given, the moment that something forbidden gets mentioned.
Okay. Now think about the consequences.
Yes, there have to be consequences. The protagonist who gets away with nothing bad happening to him is A Problem (see point 5). At worst, he’ll be one of those people we’re supposed to cheer for because he didn’t fall for tradition, as in point 2. At best, there’ll be a convoluted explanation that may work for the scene it appears in but invalidates all the build-up that the author has done. You don’t let tension just go “poof” like that.
So. What are they?
Madness is a common one. H. P. Lovecraft does that pretty dang well, though in that case there may also be madness before the fact; anyone who can do what Charles Dexter Ward did has a pretty clear case of the crazies even before they manage to conjure an Elder God. It has the nice aspect of tapping into a common fear, and also of permanence. If an author turns a protagonist mad, she doesn’t often heal him as quickly as she does of a wound.
Death is also common, but probably unlikely to occur in the case of protagonists. I regret that, because I deeply admire the courage of someone who draws the Sword of Dralcur knowing he’ll die—and then does die, instead of being saved at the last moment. Also, if there are detailed nasty stories about what messy things happen to people performing the forbidden action, make the death messy, please. No last-minute saves for that, either. (Point 5!)
What others? Well, there’s physical transformation, whether into a monster or into another being. That’s always neat. There’s the unleashing of a guardian beastie or demon that hurts and kills other people instead of the protagonist. There’s the freeing of an enemy that hunts down the protagonist wherever he runs, no matter how long it takes. There’re other kinds of slow and relentless doom, such as disappearing a piece at a time.
I’m sure you can come up with even worse ones. And feel free. Someone made this thing forbidden for a reason. What was the reason?

5) No flinching.

This is a continuation of Point 4 more than anything else, but I thought I’d mention it separately. Even if the author has a nasty fate all ready to unleash on the protagonists, she’ll turn it aside somehow. “Oh, the guardian beast died for want of food.” “Oh, the poisoned arrow jumped at him, but he was wearing armor and it clanged off.” “Oh, he would have gone mad, but he drew the Sword with a pure heart, so he didn’t.”
This is silly, especially because of what I suspect about its origins. It’s not that the author is unwilling to harm her characters; often the books that have forbidden things in them also have high body and angst counts. It’s not that the author lacks imagination; she came up with the nasty consequence, after all. It’s that she Wuvs the protagonist, and knows what kind of permanent damage or insurmountable obstacle the consequence would inflict, and while she might be able to stand the thought of him in grief and guilt and pain and unrequited love, she cannot stand the thought of him dead or mad. Or pursued for the rest of his life by an undead hound that will corrupt his flesh with a bite, for that matter. So she dissipates tension not by having a silly consequence or nothing at all behind the forbidding, but by bending the magical laws of the universe for her protagonist.
Now, perhaps you can manage this successfully—but in that case, the book is as much mystery as adventure story, with a whole alternate explanation for the protagonist’s survival that runs along under the surface of the novel and which no one picks up on and which is understandable after the fact. It takes skill, certainly more than just to claim that due to Obscure Magical Law X or Coincidence Y or Deus Ex Machina Z, the protagonist can do forbidden things and not have them rebound.
When I read one of these endings, I want to make the author sit down and read Lovecraft until she gets it through her head that nasty things can indeed happen to protagonists who go meddling with things they shouldn’t, and it’s great.

6) “But as you REAL, educated readers will know…”

This is a corollary to point 2. Yes, that means I still hate it.
I don’t mind stories that blend fantasy and science fiction. I hate stories where the characters are made to look stupid because the forbidden thing is actually a technological artifact or a relic of twentieth-century-science or whatever, and the author is giving the readers a chuckle because of outside-the-story knowledge—artificially inviting them along for the ride while leaving the characters in the dark. “Ha-ha! How stupid they are, not to recognize that their big forbidden machine is actually a computer!” or “Ha-ha! How stupid they are, because they don’t realize that the world isn’t flat!” stories wore out my patience long ago. The author set up her story so that her characters would have no option but to fail to recognize the technology or truth and come to an odd conclusion, and then she’s blaming the characters for it?
I prefer other-world fantasy. I prefer fantasy that has no “cutesy” scenes of people misinterpreting mailboxes and computers. I prefer fantasy where everyone on the planet does not turn out to be the descendants of ancient spacefarers. These ideas, like the trappings of point 3, are not bad when something’s added to them, but authors show them off as if no one has ever done them before. YES, THEY HAVE. Now do something to show me why I shouldn’t throw the book across the room when the “forbidden” cave complex turns out to be the sewer system of New York City.

7) It is fatal to explain everything.

At least, if you want any sense of mystery left, it is. If your character explores every secret of that one cave, leave others that contain unexplored secrets. If your character has to go down one dark tunnel and rouse the spirits of the dead, then leave one locked door that we never get to see behind. (Hi, Tolkien!) If the Elder God comes out and devours someone, then don’t explain the arcane meaning behind every symbol used to summon it and every detail of its natural habitat and life cycle.
Why? Because while you might have to have things be knowable—many authors really, really seem to hate the idea of “things man was not meant to know”—to make them known sacrifices mystery. I am scared of spitting cobras. On the other hand, I know what they can do and how to avoid them. If I know the same thing about the bogeyman in your story, then I’ll only be scared of him the way I am of a spitting cobra—and I’ll always remember that this is fiction and be able to emotionally disengage from it.
I suppose that, for me, the sense of the unknown forbidden, the tower or the mountains no one ever comes back from, is more powerful than the sense of the known forbidden, the werewolves that are going to devour you if you go into their woods. But even the known forbidden can have its power drained if you explain everything down to the last perfect detail, or, deities forbid, make the werewolves misunderstood gentle forest creatures. If you choose the forbidden as a sense of power, don’t weaken yourself.
That was hard. *hides under bed*