Step 1: The Terrain:
All that horror goodness has to happen somewhere, and that somewhere needs to be JUST annoying enough to ruin people's day, but not annoying enough to detract from the main event.
Luckily, the oft-overlooked basic traits a marsh
do a lot of the work for us, limiting vision range and making everything hard to move in.
Charnel bogs are your normal swamp, but Evil (with an E). Spellcraft check or fizzle your good/healing spells, auto-fatigue when resting, no Good spell recovery, and no HP recovery from resting. Perfect for wearing down our victims.
While not relevant to the average adventuring party, the Charnel Bog also has an uncapped escalating will save that's effectively a save or die (permanent amnesia, apathy, and despair that causes the character to give up on the adventure), but it's a DC 10, happens at the end of a WEEK, and goes up by 1 per week. Good for littering the area with corpses of NPCs and picking off people ravaged by disease though.
Step 2: The Hazards:
Swamps are famous for being inimical to any life not native to them (Charnel Bogs even moreso). To this end, we introduce the wonderful world of inhaled diseases. Remember that saves vs Disease can be rolled secretly, which means you basically have free reign to hand diseases out when a character is completely immersed in an inhaled disease environment for extended periods of time.
Cackle Fever: (SRD) Symptoms include high fever, disorientation, and frequent bouts of hideous laughter. Also known as “the shrieks.” (1d6 Wisdom Damage/day)
Mindfire: (SRD) Feels like your brain is burning. Causes stupor. (1d4 Int damage per day). Unclear if they mean Stupor as in the spell effect (which is basically a walking coma), or it's just flavor.
Life Blindness: (BoVD) Lose the ability to perceive living beings (Even plants!) in any way (treating them as invisible, silent, and odorless). The isolation this causes makes the victim completely antisocial, inflicting the effects of the Emotion(Despair) and Emotion(Hate) spells.
Remember, Diseases can exist in the same space at the same time, and with the effects of the Charnel Bog chipping away at the resources of the party, Diseases are sure to make them even more vulnerable to what comes next:
Step 3: The Vegetation:
With any good swamp, we are going to need the grasping vines and raking trees that make up all of a swamp's vegetation. To this end, I present to you one of my favorite "barely a creature" creatures: The Boneleaf.
Despite looking like a copse of vines and trees, the Boneleaf is actually an Aberration (with the plantlike traits ability). It has an Int of 8 and a wisdom of 16, making it clever enough to use its Illusionary Lure ability to cause serious "lost in the swamp" hallucinations.
Illusory Lure (Su): ...as the spell major image (7th-level caster), except that this illusion has a duration of 1d6+1 rounds, and a 1d4 round recharge. DC 14 Will save to disbelieve.
While the save DC is trash, it doesn't have a per-day limit, and between the Charnel Bog's fatigue and wisdom damaging diseases, the boneleaf has the home turf advantage.
Pervasive Sentience (Ex): ...If boneleaves are present in a given area, there are always 1d6+6 of them in the region. They are considered solitary creatures because each boneleaf in the group remains anywhere from 100 yards to a mile away from its nearest fellow...
...Anything one boneleaf experiences is known to all of them, so characters who encounter a second boneleaf after defeating the first will find it anticipates their tactics and abilities.
Put these two abilities together and you have an opportunistic ambush creature who has to be killed between 7 and 12 times, learning the party's weaknesses as it goes.
While it says the Boneleaf uses its illusionary abilities to mostly make small things like treasure to lure targets, it's entirely valid for it to create more elaborate mirages like a safe space to set up camp. It can cast it multiple times, so there's no reason it can't recast the same image, with minor changes each time as it slowly deprives the party of resources. Imagine setting up camp, setting down a pack, and the pack being replaced with an illusion...Major image hits all the senses, so there's a lot of wiggle room here.
Step 4: The Denizens:
While there is no shortage of things that can be found in a swamp, there are a few that stand out as horror trope all stars.
The Bog Imp has crap stats and almost no combat potential, but it has 3 abilities that make it brutal in the environs of a horror swamp:
Bog Imps can use their burrow speed (60) in place of swim speed in any liquid more viscous than clear water.
This combined with their natural amphibious nature makes them very challenging to catch for the average adventurer. No stagnant water nearby? Never fear, they bring their own!
"Any nonmagical liquids that comes within 60ft of a bog imp immediately stagnate"
This includes things in containers too, as it mentions making the party supplies nonpotable. Consuming the liquid forces a Fortitude save vs 1d3 HOURS of Nausea. The Nauseated condition is a DEATH sentence in almost any situation.
Nauseated creatures are unable to attack, cast spells, concentrate on spells, or do anything else requiring attention. The only action such a character can take is a single move action per turn.
You know what would really suck when you are surrounded by stagnant muck and puking your guts out? Getting dragged to a murky grave.
A bog imp can, with a simple glance, cause anyone standing on or in water, mud, or any other nonsolid surface to sink...the target is dragged beneath the surface by a spectral force...Once beneath the surface, the target is subject to drowning
Will save to negate, takes a few rounds to finish pulling a person under, and others can help get them out with a DC THIRTY strength check (which only resets the sinking timer, not actually end the effect) Note that there is no way for the actual target to resist once they fail the save. It's effectively a Save or Die (Eventually).
Drowned (CR 9 Undead, MM3 pg 46)