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Setting fire to a container full of gunpowder can cause it to explode, dealing fire damage to creatures within 10 feet of it (3d6 for a powder horn, 7d6 for a keg). A successful DC 12 Dexterity saving throw halves the damage. Open Game Content (place problems on the discussion page).; This is part of the (3.5e) Revised System Reference Document.It is covered by the Open Game License v1.0a, rather than the GNU Free Documentation License 1.3.To distinguish it, these items will have this notice. If you see any page that contains SRD material and does not show this license statement, please contact an admin so that.

Patches of yellow mold are most commonly encountered in dark, damp locations, and grows in 5-foot square and larger patches. If disturbed, the yellow mold releases a cloud of spores in a 10-foot radius around itself. Creatures caught in the spore cloud take 11 (2d10) poison damage and must succeed on a DC 15 Constitution saving throw or be poisoned for 1 minute. The creature takes an additional 5 (1d10) poison damage at the start of each of their turns and can repeat the saving throw at the end of each of their turns, ending the effect ton itself on a successful save.

Yellow mold is instantly destroyed by fire damage or sunlight.

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:

Brown Mold:
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.

Commentary:
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.

Green Slime:
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.

Commentary:
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.

Yellow Mold:
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.

Commentary:
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.
Rot Grub:
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.

Ear Seeker:
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:

Memory Moss:
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.

Commentary:
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.

Statuary Ivy:
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.

Commentary:
It’s a cockatrice! Fspassengers fsx crack.

Yellow Musk Creeper:
As per yellow mold, albeit in the form of a rare and trop

Dmg Fir Alcohol And Fire 5e