I know, it's counterintuitive. "Isn't this a horror campaign? Shouldn't I be scaring my players?" I hear you ask. No. Your best bet is to aim for a slowly growing unsettling feeling. Tension is your best friend, not jumpscares(which won't work unless you're playing in-person. If so by all means use jumpscares). Whether you're using grotesquely descriptive gore, or psychological horror, or eldritch horror (like Call of Cthulhu), take it slow and get deep into the grimy details.

I know literally above I said that you want to make your players feel uncomfortable and unsettled because you probably won't scare them. BUT that doesn't mean you get a free pass at being insensitive. You never know what kinds of lines your players have. Torture, sexual harassment and assault, and domestic violence are a few common no-goes for people, in my experience. It never hurts to ask! That also lets your players know that a) you're respecting them (which means never, ever, EVER cross a boundary they have explicitly put in place. They'll immediately lose trust in you, believe me) and b) it can also serve as a warning that everything else is fair game. If they didn't say that tons of gore is something they're not comfortable with, then by all means use it.

In other words, make them uncomfortable by making nothing familiar (unless you're taking it away for plot, of course). For example, say someone knew what vampires were and how they worked in 5e. As long as you're not setting them up with the expectation that everything will be done by-the book, then put in a little plot twist. The horde of vampires that should take acid damage in running water turns 10 times scarier when the players suddenly find out that they can actually swim and shooting at the vampires with magic attacks from across a river isn't going to work as well as they thought it would. But you gotta keep it balanced, of course.

Make sure everyone has a knife ready to stab into your players' backs, make sure there's no one for them to turn to. No one and nothing is safe. But most importantly: make sure you're doing this without being mean. You're just gonna end up with disgruntled players instead of a good horror vibe.

Get nasty. Get descriptive about the way the iron of the blood pooling at their feet hits their noses like a freight train, the way the pus of an infected cut oozes out of the wound, forming a disgustingly yellow crust around its reddening edges. The devil's in the details (I know I'm using that phrase wrong but whatever). Get to know your players' comfort levels first, though (See 2). Again, you don't want disgruntled players. (See 6).

What's an ant to a God? You don't even have to get the players involved in fights to do this, I don't think. Death can and should follow them around at all times, but make sure there's a reason for the deaths. Random deaths are pointless and they can and will bring down your players' faith in your ability to keep with the theme. And don't lure the players into traps designed to get them killed. That's railroading at its deadliest. (See 10).

I find that sight actually isn't the best sense to engage in a horror campaign. People are used to describing things visually. This is normal, this is comfortable. And remember! You want to avoid giving the players a sense of security. So use other unconventional "out of sight" senses. It's one thing to say "You see the flesh of your hand dissolve in acid," and another thing entirely to say, "The first thing that hits you is the smell. Acrid enough to make you gag, you can feel bile coming up from your stomach as you watch the flesh of your hand sizzle into nothing in the acid." Smell, touch, sound, and even taste (if appropriate) can be incredibly valuable tools.

Another thing, think of horror movies (if you're running a horror campaign, I'm assuming that's your thing). (SPOILERS! sort of FOR THE BLAIR WITCH PROJECT MOVIE) The most effective horror comes from a threat that is never seen. Personally, The (original) Blair Witch Project chilled me to the bone, and we never even saw the witch! We heard the anguished cries of the protagonists, and felt lost right with them, without seeing a thing.

What kills a campaign more than busy schedules or a railroading DM? Upset players that are upset not because of an effective and emotional game, but because they have an insensitive DM. I've heard horror stories (heh) of people who have reached out to their DMs saying "I really don't like a, b, and c, for x, y, and z reasons" and the DM responding with "Okay, ~fOr ThE sAkE oF tHe PlOt~ I'll put exactly that into the campaign." No. Stop that. Bad. That's not good. I already said this in no. 2 on this list but it's so important that I have to say it twice. Sure, everything else is fair game, and the best DMs are the ones that can get creative with what they can do, not the ones that get upset with what they can't do. Even if your players come to you in the middle of the campaign saying "hey, I'm uncomfortable with a. Can you maybe try something else?" L I S T E N  T O  T H E M. They'll end up leaving if you don't. There are plenty of other things to do!

This kind of falls under no. 3. Don't make things predictable! Don't back them into a corner, but take away all the bushes for them to hide behind. Again with the vampires, if the vampires want to suck your blood (bleh ble-blehh), maybe they do it not because they need blood to survive, but because they're collecting blood to feed a much, much bigger, stronger, scarier thing. Now you've turned some vampires just trying to survive into sinister vampires with a potentially dangerous and certainly unpredictable agenda.

At its best, it gives your players information on exactly what they need to do (familiarity, that's a no-no) and at worst it can destroy a game. Best way to circumvent this is something I saw called the Three Clue Rule. It was targeted at Murder-Mystery style campaigns, but it still definitely applies to everything else. Present your players with a scene, and put at least three clues (objects, things NPCs say, etc.) in it, and let your players loose. Say your party was looking for the secret entrance to the underground hiding place of this vampire cult that has been terrorising the town and sucking villagers' blood dry in order to feed this eldritch abomination that they call their leader. Maybe put half a trail of blood into the woods in the right direction of their hideout (clue no. 1), a letter from the loved one of one of the victims detailing what they say when they found the body, detailing how it looks like there aren't any teeth marks but actually a single puncture wound where they put whatever they're using to collect the blood (clue no. 2), and farmers on the outskirts of the village complaining about small, periodical earthquakes that are "scarin' my cows!!" that's actually the infantile heartbeat of this eldritch abomination of a fetus growing underground (clue no. 3). Not only have you automatically given the players at least half a village, a forest, and a few NPCs to explore and question, but also the solution is no longer hidden behind them having to know one piece of information that could easily have been missed by an inattentive or distracted player or a failed Perception check. You now have three of those. Which means, I also want to add, that the more clues you have, the better! Worst case scenario, you have a plan B, C, D, E, F, G, H, and I ready for when the players (inevitably) get a nat 1 on a Perception or Investigation check, and best case scenario you have players that are super proud of themselves for putting the pieces of the puzzle together and advancing.

Also, be open to creative solutions and spur-of-the-moment opportunities for stuff. More than once have I gotten ideas from my players' anxieties (a howl from a small pack of dire wolves I was going to have the party fight turned into a much more deadly werewolf when the wizard suddenly asked what phase the moon is. I didn't know it was full until that moment).