I am currently designing my first horror campaign and what I've come up with so far is pretty dark and gruesome. I worry that if it is gut wrenching the whole way through, eventually the players will either mentally adjust, get bored, or pass their tolerance/exposure level. How can I stop that from happening?
Building Tension to Build Horror
I’ve played RPG’s since I was a kid, but I’ve also written narrative as a hobby for years. RPG’s combine the intellectual stimulation of board games with the deep engagement of storytelling (books and films). When the players calmly intellectualize their attempts to WIN, they’re in board game mode. When players FEEL something, they’re in engagement mode. As you’ve already guessed, horror can only be experienced by engaged players. Players experience horror as a form of tension. Tension engages them laying the ground work for horror.
The concern your question addresses is The Law of Diminishing Returns. For example, players may find zombies scary at first (a form of tension), but if they keep running into them, the zombies soon become familiar, then boring, and eventually comedic (a total lack of tension). Consider the zombie meme. It began with the fearful “28 Days Later” but was quickly played out by copy-cat films and computer games eventually producing “Shaun of the Dead” - a 180 degree shift from horror to comedy. Further, almost everything is subject to the law of diminishing returns. As the other answers suggest, a campaign seeking to remain tense by existing within the horror space for extended time, must tap the needs of tension to counteract the law of diminishing returns. Specifically, it must push high-stakes dilemmas on the players to build and sustain tension.
A dilemma is a difficult choice for the players. For clarity, a dilemma is more than a decision, which players make all the time. “Do I fire an arrow or cast a spell?” In contrast, dilemmas are the tough ones, coming in only two forms; lesser of evils OR irreconcilable goods. An irreconcilable good is when the player/s desperately WANT all of the choices, but they can only have one. This works great for treasure rewards in gaming by the way. Lesser of evils is when the player/s passionately HATE all of the choices, but MUST choose one. Needless to say, a horror campaign benefits greatly from lesser of evil dilemmas. With this focus on dilemmas, consider the following.
Pacing is not the scale of an event, it’s inherent drama, or the speed of travel. Rather it is the frequency of dilemmas over time. Many gaming sessions feature a battle-royale taking hours. Counter-intuitively, this can present as a slow pace. Why? Because of the lack of dilemmas while the battle transpires. Effectively the players make one choice, to fight, then play out that choice for hours. In contrast, a fast pace might look like this:
GM: A hoard of zombies surround you, closing in, but you notice a stair descending into blackness. Vapors seep from the opening. What do you do?
PLAYERS: We don’t like the look of those stairs, but there’s too many zombies. We’ll have to chance the stairs.
GM: You rush down the stairs, barely able to see. The sound of pattering feet signals the zombies pursue close behind. The stairs stop before a pit, water rushes by at its bottom, barely visible in the blackness. If you drop your backpacks you could leap across. What do you do?
PLAYERS: No way we’re dropping our stuff and we still can’t fight the zombies, but who knows what’s in that water. Damn, we’ll have to swim for it.
GM: You plunge into icy blackness, the water sweeping you into a large cavern. You barely make out zombies crowding either shore. As the current races you along, something slithers past your paddling feet. What do you do?
This quick succession of lesser of evils dilemmas constitutes a fast pace. Pacing, fast or slow, is not horror itself, but a fast pace creates tension priming the players to feel horror conveyed in other ways.
Many of the other answers detailed suggestions for establishing mood so I will not elaborate save to observe that mood can target the players (for instance, dimming the lights in the room) or the characters (descriptions of gore in the game setting). For maximum effectiveness, use both.
Alluded to in your question, rhythm is variation in the pacing and mood. Since both pacing and mood are subject to the law of diminishing returns, a GM must vary both to avoid draining the effectiveness of either. I’m sure you’ve heard that movie description ‘roller-coaster ride of excitement.” This references an effective variation of rhythm. Just remember, when varying pacing and mood, avoid repeating patterns. If the players detect the pattern, they’ll predict the surprises because the pattern itself is subject to the law of diminishing returns.
Remove Safe Ground
Like mood, this tension-building technique can target both the players or their characters. Don’t provide the CHARACTERS convenient ‘home bases’ (such as a room with a door they can lock) to sleep or heal in, where they can control their safety to turn off threats. Even more important, deny the PLAYERS safe ground. Avoid quantifying opponents and environment when feasible as this allows players to intellectualize their odds of success, taking emotion out of the equation. Also, it is common for GM’s to allow players the ability to stop game time while they tackle dilemmas. This kills the tension horror demands. I often employ the prop of a cooking timer to drive home the point. “Do you fight the zombies or run down the dark stair? You have 10 seconds to decide. tick, tick, tick, tick...
Easily the most common way game masters defeat their own efforts to raise tension is by making monsters fight to the death; for once monsters are destroyed, relief replaces tension allowing players to relax. These easy victories also sabotage the players’ sense of triumph. Consider a pack of undead wolves pursuing the players, employing hit or run tactics. Met with strong resistance, the wolves melt into the forest, but the threat remains. “What happens if the wolves attack while we’re fighting the zombies?” If the tension starts to wear off, the wolves attack again, or maybe it’s enough to hear rustling in the bushes.
My point in all of this, is that horror requires the players to FEEL tension as opposed to dryly KNOWING that a monster is there. When the players know what’s going on, they feel in control, and mentally switch into board game mode. Pacing, the removal of save ground, and denying closure build tension. Variations in rhythm prevent the tension from wearing off allowing mood to transform the tension into horror.
The key to a good horror game/movie is the pacing; if the characters are constantly in peril and exposed to horrific things then it will become bland. From my Cthulhu games I've found keys to this are:
Any sensation, experienced in full without stop will eventually allow or force those who experience it to adjust to or move away from it. For a 'horror' campaign to be effective, memorable, and successful for a good duration of time it is necessary to have a solid understanding of two things, everything else is secondary and dependent on your performance skills. If you understand the two things, then your campaign can rise above poor performance skills. If your performance skills are solid, then you have a chance to go for a legendary campaign the players will discuss for decades.
1. The meaning of fear
We use the word horror to describe this genre, but all the words we have for fear have different meanings and describe different sources and sensations. This affects the games we run in both how we describe scenes and in how we plan them. Consistency in design will allow you to help you lead your players in the direction of horror. Be aware that you can only go for the physical jump, shock, or reaction a limited number of times, and that this may not be the biggest scare that your set-up can generate. Shaping the session so that you can get the fear in under their skin will hit them harder, and be remembered longer. By building through the stages of fear and the little things which produce it, your sessions subtly shift and grow in intensity, preventing things from becoming stale.
Know what effects you are actually trying to accomplish. For example, it is important to know that gore does not equal horror or automatically produce fear, particularly when imagined. As a long-term Call of Cthulhu player, I have written about this on my blog to some degree and would like to recommend this link on Fear, and this link on running long-term horror campaigns from my blog plus this excellent essay from mxyzplk as supplements to my answer below.
2. The players scare themselves
A horror game requires a certain degree of acceptance on the part of the players. All the efforts in the world to create mood will not counteract the person who opts to destroy the mood. It is far, far easier to destroy than to create. This acceptance does not require that the players want to be scared by the GM, it just requires that they want their characters to accomplish a goal or goals within the scenario despite the obstacles in their way. As the players are really in control of their own fear, leading them to it also works to prevent boredom or desensitization.
Knowledge of the Audience
Fear Breeds Fear
This may all sound very hard, but it is something most of us do all the time when we want to promote empathy in the listener. You want a certain amount of familiarity and repetition in the tale to allow the listener to make sense of things and draw conclusions. You want just enough ambiguity and variation to make them question their choices and actively put themselves in the scene. Reflect the fear you want your players to feel, let them fill in the blanks with their own experience, enhance the sense of a shared experience, and give them enough rope to hang themselves.
When they are doing the hanging, the players do not get bored~
You cannot sustain a horrific setting all the time. Thus pacing is key. Allow the players some normal and fun times -- both IC and OC. However, there are a few things you can do to always unsettle them. Mostly they boil down to alienation. You want a disconnect between nice comfy reality and the game world. You want to mess with players' minds -- just enough -- so they feel their heart rate quicken.
Music: Horror soundtracks (Silent Hill, Biohazard/Resident Evil, The Thing to name but a few) are good but so are others (Delirium and Ugress for example). The trick is unsettling -- beat changes slowly but surely. Have a particular sound track for particular event types so you breed anticipation in your players. Every time song X plays, something bad happens. Then not all the time... But you can bet your players will get paranoid.
Sounds: Do you know the sound of flesh being torn from bone? No, get a whole chicken and strip it. Now, tape this. Play it when you describe cultist A ripping flesh. There are a zillion good sound bites on the net: rain, storms, screams, gunfire, baby laughing. The latter is wonderful to play in the cultist's lair amid horrible things. Creepy.
Light: No more bright lights. Use a red filter on your one lamp -- I used a red sheet on a overhead projector or a black light. Of course, no outside lights etc... Yes, this is bordering on sensory deprivation.
No food, drink, or other distractions. Oh, that does include heating. Being a little cold makes you a little on edge.
Scare the player(s) What are they afraid in real life? Can you use that in the game? Do make sure you get player buy-in on this and use common sense. You do not want to lose friends over this.
Wear dark cloths: No flowery summer dresses here! No clever XKCD shirts. Just dark cloths. You can even use subtle make up.
Voice: Vary your speech. Speak softer then slowly get faster. Build anticipation and then drop it suddenly with nothing happening. Or build it up to something horrific.
Learn from scary media: The Kingdom (the Lars Von Tiers one) is one scary show. You could learn a lot just by watching it (in the dark, at night, alone) and using some of the tricks that they use there. But look at what scares you: Can you use that? I bet you can.
Academia: fear is a field of study. What causes it, how to deal with it, and how to create it are all described in many papers. Go look for those!
A side note, your players will rebel at these. They will need breaks to relax, to joke around, to just breath. Let the characters have that as well.
Think about a horror movie for minute.
Most of all (like with any game), know and watch your players. Keep an eye out for signs of boredom, tolerance etc and use those moments to quicken the pace and kick the plot in the direction you want to take it. If you find that your players aren't as scared as you think they should be at a given moment, bump up your description, your pace, or give them a fright (have a scare prop or two on hand just for these occasions).
I have found that the most effective way of showcasing horror is to contrast it with comedy or general happiness. À la Disney's Cars, "Sometimes you have to turn left to go right", the easiest way to set up a horror is with a bright and sunny scene with happiness and joy, then crush it to a pulp in front of the PCs. This doesn't work consistently, but is excellent in short bursts. For instance, if Cthulhu shows up to a game set in the grimdark horror future it's expected, but if Cthulhu shows up to the reception at your wedding screams will ensue.
Novel experiences are by definition anathema to boredom. They're new, and for the curious, they're interesting. If the giant monster stops chasing you mid-stride (for the first time) it generates curiosity. Curious people investigate. Shocking the curious is a way to progress both plot and action. Careless people also drive plots, occasionally. There is also the party sacrifice to create novel experiences. If someone has to be the "bait" you are sure to create a bit of excitement, especially for the chummy bait.
Talk About It
(Staying Within Players' Tolerance)
The safest way to ensure that you do not overload your players with abject scenes of horror, disgust, fear, and loathsome scenes is to talk about what's going to happen before hand and set limits. Good sense also plays heavily here. If it's super taboo in your culture, don't throw it into your game unless you have very specifically received verbal and/or written permission to approach those subjects. Subjects that can generate fear, but may not be necessarily taboo in American culture may include:
If you do decide to approach the latter mentioned subjects, please leave room in your game for players to excuse themselves from play or call an end to the session. It is more important to maintain friendships and acquaintances than it is to scare them, disgust them, or make them want to leave your table and cast you out of their circle of friends.
Subjects that will most likely be intolerable
(Warning! Get permission, get permission, get permission!)
These things would also most likely be taboo: