Hey all, me again, I'm returning to pique some minds about some psychological effects that players could see/hear/experience to promote a spookier setting. I'm running a weird west campaign that involves a lot of maddening effects that players run into based on the situation. For my upcoming session, my players will be entering a marsh/swamp area which is being inhabited by a green hag who has been mutated to the point of madness. I plan on having them make checks at various times to see or hear things that could promote a scarier situation along with beating them down a bit before a bigger battle. I have a few ideas like making them see a grotesque version of themselves in the swamp water, or seeing sickly spirits that have been trapped within the bounds of the swamp. I was just wondering if any one else had some interesting ideas for things like this, not even necessarily swamp-related. Anything would be appreciated!

One fun thing that will blur the line between IC and OOC is to, at regular intervals, ask the party to roll perception checks. No matter what they roll, say something along the lines of, "Hmm, okay. Neat," make a note in your notebook, and keep on with the scene. Then do it again, five minutes later. Then again, ten minutes later. In reality there's nothing, but it'll drive your players to fits of paranoia. This is a metagame way to simulate the feeling of "being watched," or the sense of "impending danger." 

Other ideas:

• They encounter will-o-the-wisps - not necessarily the actual DnD monster, but that's certainly an option. It could also simply be that they see the lights of lanterns in the distance, perhaps accompanied by faint voices, but when they try to investigate it seems they're never able to catch up with the lights - no matter how fast or how far they go, those mysterious lights always stay the same distance away. For extra creep factor, have them go in circles; perhaps they find themselves by one particularly large, gnarled, and notable tree again and again, even though they could swear they were going in a straight line.
• Trees that bleed when you cut into them.
• Don't forget to describe the smells they encounter. Smell is such an underutilized sense, but one that's so effective! And a swamp especially is ripe (pun intended) for all sorts of gross and creepy smells.
• After wandering the swamp, perhaps they encounter a friendly and helpful-seeming person - maybe one who offers to guide them through the swamp. The person is perfectly polite and will genuinely help them get closer to their goal, though they refrain from participating in any combat the party may encounter. They'll help the party make camp for the night, but come morning they're gone, just like that. A search of the area will reveal nothing but a skeleton half-buried in the muck and the roots a short distance away.

Mirages for the desert area, and tell them that they fell like they are being watched. Have the players roll perception, but no matter how high they roll, they find nothing.

Tell them they hear the sounds of people around an evening’s campfire such as low laughter and indistinguishable conversation in murmured voices, they can smell the heavenly scent of meat roasting on the fire mingled with woodsmoke, feel its warmth from behind the trees, they can even see the light shining. But when they round the corner... nothing is there.... 

Quote from Nat_30 >>

• Don't forget to describe the smells they encounter. Smell is such an underutilized sense, but one that's so effective! And a swamp especially is ripe (pun intended) for all sorts of gross and creepy smells.

 Nothing puts players in the PCs shoes more than telling them what they smell in as close details as possible.

Giant spiderwebs, with people tied up in cocoons. And one of them is still alive.

When I think of maddening effects, I go in a few different directions. Typically, when I seed paranoia into a campaign, I use a divide and conquer strategy, as it's easier to use these sort of things when you can't constantly see the other players. Using that tactic typically requires either an NPC that the party trusts or a Player that you trust, during which you create the idea of an imaginary possession or fake doppelganger, which brings in a certain lack of trust. Throw in a hallucination or supernatural effect or two and you have a party that's aimed at the throats of at lead one of their comrades. However, seeing as to the fact that this might not be a viable option most of the time, I typically like to reflavor psychic damaging spells, Maddening Darkness being my favorite (my current favorite reflavoring is that it's a huge torrent of blood, the rushing of the liquid and it's opacity simulating the darkness, and the taste and other details representing the particularly psychologically damaging aspect of it). Using that spell in particular might not be the best idea, given its power level, but tapping into phobias to describe psychic damage and the like is typically something that sets people on edge (trypophobia and claustrophobia are the ones that bother most people, so the more vivid imagery you can create of those, the better).