Okay, so we all know the old standbys: the green slime, the yellow mold.
Here are some more, from a variety of sources and at my whim.
First, the DMG:
Drawn to warmth, a 10 foot square patch of brown mold makes frigid the air within a 30 foot aura.
When a creature starts or enters within 5 feet of the mold, it takes 22 (4d10) cold damage, constitution save DC 12 for half.
Brown mold is immune to fire, and bringing fire within 5 feet spawns a new 10 foot square of the mold around the fire. Exposing it to cold damage instantly destroys it.
What are we to make of this? Brown mold deals cold damage, nigh definitionally. Its fimbulwinter-like effect is presumably intended to be constant (it’s not that it deals cold damage reactively to creatures, but that the area around it is always polar, and creatures are exposed to it). I suppose it inhibits its own growth, or has some annular ring around which it does the cold leakage.
Quasi-naturalistic though “this mold is cold” might be, the actual game rules feel to me like they work better for an unstable connection to the Ice Plane than as an actual creature, since as I say, it should self-destruct.
Premodern refrigeration technology? Quite dangerous, but maybe you’d use a spore-fine mesh and a fan, or a captive-fluid exchange manifold to limit exposure. I kind of like that idea — light the boiler to make some ice cream.
Acidic, sticky and hungry, green slime eats everything but ceramics and stone, which it clings to in any orientation.
A 5 foot square of slime drops from walls and ceilings onto creatures whose movement it detects below it. A creature aware of the patch can make a dexterity save DC 10 to avoid, but otherwise splat. A creature touching green slime takes 5 (1d10) acid damage on contact and again at the beginning of each turn. It deals double damage against wood or metal. Any scraping tool which removes the slime is destroyed (unless it is ceramic). Sunlight, effects which cure disease, or any amount of cold, fire or radiant damage destroys a patch of slime.
An oldie but a goodie. We’re missing phrasing about what happens to the victim of a scraping (in my opinion, they share any damage dealt with the slime!), any chance of the scraping failing (I’m okay with that), and in my opinion the damage is lower than it could be. I suppose I could always throw in Malachite Slime and Emerald Slime when I feel the need to up the damage, though!
Worst of all: green slime is real.
Sporous and dark-loving, yellow mold fills a 5 foot square. Touching a patch fills a 10 foot cube dealing 11 (2d10) poison damage and poisoned for one minute, constitution save DC 11 negates. While poisoned, take 5 (1d10) poison damage at the start of each turn, saving again at the end. Sunlight or any amount of fire damage destroys yellow mold.
This seems fine, but a little inconsistent. I’d add radiant damage vulnerability and effects which remove poison and disease, as with the green slime.
This also implies an inhalant poison, Preserved Yellow Mold Dust. Harvested at great expense, when it reconstitutes in the mucous membranes of an individual, it’s super deadly. 250 gp, Inhaled/ingested, effects as per exposure to yellow mold spores.
As far as I can tell, the only 1e nasties which 5e is still missing are the rot grub and the ear seeker.
The rot grub is a tiny flesh-loving parasite which spreads by touch, generally the touch of a corpse. Avoiding exposure requires a dexterity save DC 10, resistance to weapon damage, or specialized clothing. The round after exposure, the grubs can be removed with a DC 15 Medicine check with a knife or fire (1d4 damage per attempt if the normal damage would be less; fire damage dealing more than 5 points of damage is automatically successful even if the medicine check fails).
Any effect which removes disease kills the grubs.
Otherwise, at the conclusion of each short rest, the infestee must make a DC 10 constitution save or becomes incapacitated with pain for 1 hour, suffering 1d6 damage at the beginning of each round they are incapacitated.
I don’t like ’em. If you really want an ear seeker, just use a rot grub and have it go for the ears. It’s a rot grub that lives in doors.
Some more dungeon hazards which I like, adapted from the pathfinder SRD:
A 5 foot square of black moss, growing in grottoes and caves or warm, moist climes, near settlements or lairs. It ripples gently when exposed to light, and has open starlike petals if it hasn’t fed within 24 hours. A creature within 60 feet of a patch of the moss feels their mind begin to wander and old emotions begin to surface. A creature within 5 feet of a patch of active moss and breathing its spores must make a DC 10 Intelligence save or lose all memory since they last time they slept and be confused for one hour, save once per round negates. Whether the initial attack succeeds or fails the moss will then go “dormant” for 24 hours while it digests. Repeated exposure causes amnesia at iteratively removed time periods.
Eating digesting moss causes the consumer to experience its contained memories, restoring them (or implanting them). Eating moss which doesn’t contain memories gives general sensations drawn across its various meals, in a sensation very like dreaming.
Cold and fire damage destroys the moss.
As originally presented, this stuff also stole spells. No way am I bothering with that. I think stealing your brain and making you short-term crazy is enough for me, thank-you. There’s a 3e poison called “Id Moss” which my headcanon makes into an application of this stuff.
I love the pen-sieve effect where eating the moss gives you a memory. Memory Moss Potions, made by plucking it at the point of freshness and then dissolving the moss in an alcoholic solution, allowing a holographic attenuation of detail and exchange of thoughts, are just so cool.
And it leaves you, maddened and amnesiac, at the bottom level of a dungeon with no memory of why and how you got there.
A tropical clinging vine whose contact poison causes flesh to lignify, easily noticeable by its silver sheen and galled, bumpy appearance (statuary insects). Each round which begins while exposed warrants a DC 10 constitution save against magical petrification. The first failed save inflicts the restrained condition. A second (if still exposed) causes petrification for 24 hours (though it is a transformation into wood, not stone). A final save after 24 hours makes the lignification permanent. A success on any of these saves removes the effect and allows the victim to break free, though reexposure requires checking again.
The oils on the ivy fade swiftly after the ivy is detached, losing their potency after 1 minute. Contact is generally limited to animals or accidental handling, since boots represent sufficient defense while standing in a patch of it and the oils fade swiftly.
It’s a cockatrice!
Yellow Musk Creeper:
As per yellow mold, albeit in the form of a rare and tropical vine. Creatures slain by the mold arise as zombies. Vulnerable to cold and fire, instead of radiant and fire.
A 5 foot patch of black mold. When exposed to flesh, it grows uncontrollably; when touched or agitated by wind it fills a 10 foot cube with its spores, exposing all creatures who enter and begin in the space. Making a DC 15 constitution save to avoid the effect. Otherwise, take 1d6 acid damage at the beginning of each round. If you are in bright light or have taken at least 10 points of radiant or fire damage since your last turn, you may repeat the save to end the effect at the end of your turn. Effects that remove disease the effect as well.
Any creature slain by the mold is destroyed, becoming a new patch of mold.
Utterly cruel! The ravenous mold can’t be scraped off, and 10 points of radiant damage is beyond the reach of early spells like sacred flame. You’re basically going to have to douse yourself in oil and catch fire, and do it quickly.
It’s a shame this stuff is acid-aspected, because this would be an interesting use for taking a wine bath to end the effect, too.
Stale and still, anyone breathing bad air is suffocating without realizing it. When you first start breathing bad air, you can make a wisdom (perception or survival) or intelligence (nature) check against a DC of 10 to recognize it for what it is; dwarven stonecutting bonuses apply; light sources dim and go out after 1 round. Otherwise you will suffocate in a number of rounds equal to your constitution bonus (as though running out of breath). This can be avoided simply by holding your breath.
Variant: Flammable Gas Pocket: As bad air, but instead of going out, open flames (such as a candle, torch, lamp or lantern) will set the gas pocket off. It deals 8d6 fire to all creatures in or within 20 feet of the pocket as it erupts, DC 10 dexterity save for half. During the round before they go out, the light sources tinge blue. The explosion is a one time event, after which the area is bad air, recharging to flammable gas after a few days.
Variant: Poisonous Gas Pocket: As bad air, but instead of suffocating, at the end of the grace period, you are poisoned and while poisoned immobilized; DC 10 constitution save at the beginning of each round negates for that round. Beginning your turn poisoned in the area of the gas deals 1d6 poison damage.
Variant: Poison Wind: As poisonous gas, but issuing from a vent, so that the air is moving. It usually has a chemical smell to it.
A 5 foot patch of shelf fungus, this paper growth forms on dirt or biological material in areas of dim light near permanent magical effects. Spellcasters attempting to cast a spell while standing within 30 feet of a patch which isn’t charged must make a spellcasting attribute check with a DC of 10 + the level of their spell. If they fail this check, the spell is counterspelled and the spellbook fungus becomes charged. Anyone touching a charged spellbook fungus discharges it; they are affected as though by a wand of wonder and the fungus is destroyed in a cloud of spores. Spellbook fungus may remain charged for up to 1 year.
A patch of spellbook fungus may be dispelled as though it were a 3rd level spell, or destroyed by any amount of fire damage.
A 10 foot patch of this greyish mold darkens the area within 10 feet around it, from bright light to dim or dim light to dark. If it is put in total darkness beyond sight of all lights (ignoring its own feeding effect), it begins to starve and go dormant within 1 round. If it spends a year dormant, it will die, otherwise reviving 1 hour after being exposed to light again.
Bringing a light source within 5 feet of the mold causes a patch to appear under and on the light source, thereafter causing the light source to produce only dim light out to the radius it previously produced bright light, instead of bright-and-dim-light. It is immune to damage from fire or radiant, spawning a new patch instead towards the damage source. It will be destroyed if dealt cold, thunder or acid damage, or cleaned with alcohol.
As noted, a darkness spell will cause dormancy.
A bright yellow cheerful fungal growth resembling a sunflower, a patch of echo fungus covers a 5 foot square. This stuff babbles continuously, repeating the loudest sound it hears within 60 feet. It is immune to thunder damage. If it is exposed to any amount of thunder damage, it begins a cacophonous repetition, causing any creature who begins or enters 60 feet of a stand of echo fungus to take 1d6 thunder damage and be deafened, constitution save DC 10 negates.
A silence effect will kill all patches in their area, as will a DC 10 perform check to “change their channel” or at least a gallon of water spread over the area.
This weeping-moss is silver-tinged and airy, filling a 5 foot square. Living creatures who smell it are overcome with despair while they can smell it, being unable to attack or target any creature with harmful abilities, spells, or other magical effects. A DC 10 charisma save on initial exposure or at the end of each round negates. A creature exposed for more than a minute may no longer negate, being permanently hopeless.
Immunity to fear negates, as does any effect which removes fear.
The moss itself may be destroyed with fire or radiant damage.