The Spider's Basin
A simple, effective dungeon trap that satisfies the requirements to be used several times and without decaying.
A rather quirky trap which is meant to be taken with a pinch of salt.
'As you enter a room, you can see that it is divided in two by a waist deep basin of glutinous brown substance which is too wide to be jumped over. Thousands of finger wide spider of an iridescent green colour seem to be walking upon the surface of the pool. The silk cocoons enveloping the corners of the ceilings crawl with a multitude of these spiders. Their rhythmic movement are captivating, but not entrancing. Analysis will reveal that although the substance in the pool is biological in nature, it is perfectly harmless. The spiders will happily ignore you while you cross this pool, dancing rhythmically among its surface, undisturbed by the ripples you will create.'
{...}
'As you enter a second room, you can see that it is laid out exactly the same way as the previous one, but this time, the substance is rather yellowish in color and gleaming red spiders an inch wide dance upon its surface. They will ignore you just the way the green spiders did, and will go about their business undisturbed by your presence. This time though, their silk looks thicker, coarser, and bands of carefully laids out nest crisscross the walls. Once again, analysis reveals the substance to be perfectly harmless.'
How it works:
This trap is devised quite simply by two elements of a very strong binary resin cutting access across two rooms. Adventurers wishing to cross first have to wade through the first basin, and then the second. Shortly thereafter, the resin will begin to set.
The adventurers will in the best case scenarios find their equipment below the waist useless, with jammed armor joint and useless shoes.
A worst case scenario would be that soon after, your adventurers will find themselves quite literally stuck to the floor amongst the skeletons of similarly lucky parties.
If you are in a high magic world, the resin could be the result of some magical fungus lining the vats, so that it cannot be magik'ed away. This would also explain its persistence after a long time. The spiders are in fact feeding on the fungus, while the fungus itself is feeding off the spider's dead remains, which is digested by the substances.
Clues
The substances should be very sticky to the touch, similar to undiluted PVA glue. On their own, they will not set however. In the second chamber, it would be very likely that some hard resin bits would be floating on the surface, from small animals or past adventurers.
Escape scenarios
It could be possible to wrap your lower legs in fabric, so that they do not stick to the floor, or each other, but the stiffness would not go away, and your legs would be in the equivalent of a permanent cast...not practical for dungeoneering. It may be possible to lay some planks/fabric on top of the resin. To counter that, make the ceiling 3-4 feet high
Interesting fact
The spiders in fact make very rare alchemical potions if killed and dried. Green spider powder is prized for its ability to remove the moisture out of the air, and is used in oils by masters of weapon alchemy to strengthen and protect priceless weapons. Unfortunately, when killed, they secrete a very unpleasant chemical, which will send other green spiders into a violent frenzy. Their bite is not poisonous, although for a spider of this size, it is very large, and will take a significant proportion of skin(diameter of a pencil), in which case, infection is likely to ensure.
Red spiders Have the ability to create a special silk, which, when a small amount is dropped into a liquid transforms the liquid into a gel. only a few grams are needed to transform several litres of water, and this can be reversed by simply heating the liquid to a lukewarm temperature. If a large amount of silk from their nest is dropped into the pool of resin, it will solidify to a consistency where it is possible to walk over it. (its adhesive qualities remain the same) which is why the red spiders will defend their nests quite brutally, since it affects their ability to feed. their bite is quite harmless singularly, but if bitten repeatedly, the poison can reach levels where the victim will begin to feel dizziness, and nausea, until they fall unconscious (death is very rare)
If the resin is solidified, the fungus will slowly digest the silk until it returns to its normal consistency.
For the spiders, the resin is nearly as solid as a stone floor, with the difference being that some bits of fungus seem to be floating on the surface from time to time, allowing them to exist.
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? Responses (10)

How do the inhabitants bypass it? There needs to be some way through for someone. Or course with magic, there could be lots of options (Freeze the goo, fly,etc).
Another escape scenerio, but would need to have some advance warning. Two pairs of hip waders per person. Wear first pair through first chemical. Discard. Wear second pair through second chemical.
These pools sound like a great place to hide blob-like monsters.

Absolutely no idea how the inhabitants bypass it. Perhaps there is a secret bypass door in the corridor between the two rooms? Or maybe the basins can be drained remotely form inside upon recognition, with somebody inside looking at the first room's entrance through a firing hole...

perhaps as an added touch, dozens upon dozens of tiny spiders crawl along the goo oblivious and unaffected by the sticking properties of the substance, adding to PCs trepidations and/or confusion.

Updated: Added the bit about the spiders
(was a great idea, by the way!)

After reading this I thought 'COOL' a chemical based trap, and then the spiders jumped out at me as cool also. Then I thought of what could be a great red herring, an oddball spider among the rest, like a slightly larger blue spider among the red, it must be important, right? it's just an albino red spider!
Good work!

Anything with spiders automatically is great, usually. The fact that you need to encounter both substances is ingenious.

Read this a while back, no idea why I didn't comment on it. The spiders are a great visual that follows a well crafted post. Sweet idea.

Great trap, though i'm going to make the mixed substances an acid that eats through leather (good-bye boots!), and then put a thorny passage way afterward (which does damage to those with barefeet)

Excellent idea that combines a few complex elements into a fairly simple to use mechanic.
I think it might be best placed into a more natural setting like a cave revered for the spiders and perhaps with rumours of a shrine or treasure at the end. The ecology is too complex for it to be easily designed but it is could be the sort of thing that evolved and was then taken advantage of.
I think it's interesting enough that I will try to make a setting to place it in specifically so I can use it (rather than use it to populate a setting).