Well, that was almost not Tuesday, wasn't it? (For some of you it will be Wednesday, but it's still Tuesday in California, so I'm good.) School's… making me really happy to have RPGs as a diversion right now. Anyway, per Simon Forster's request last week, today we have a trap in goblin territory, though any small humanoids will do the trick as well.
4: Latch/Switch Trigger
10: Ball Bearings
Description: In a room that is deep enough inside the goblin area of a dungeon for the party to believe it could be where they store their treasure there is a lever. The lever may have been built by the goblins or be a repurposed lever originally built by virtually any race. The lever is probably inscribed or labeled in Goblin by the goblins with something like "Don't Touch!" or "Ask Chief before getting some treasure!" or "Stay Out!"- anything to make the party suspect that the goblins access their treasure by using the lever. The lever has been rigged by the goblins to do two things when it is pulled: close all the doors to the room (which all open inward) and open a trapdoor in the ceiling. If any of the doors are blocked or unable to close, the lever cannot be pulled; if it is forced, the handle will break.
In the room that the trap door leads to is a large chamber filled with ball bearings that the goblins acquired from a source of your choosing; if your setting has steam-punk or Victorian leanings, they may have been stolen from an extant civilization, but otherwise the goblins likely found them or traded for them deeper down in the dungeon, as they are the remains of a lost and very advanced civilization, or else something magical. These ball bearings burst into the room, causing 2d6 damage to anyone standing underneath the trap door. (If you use minis and a mat, this is pretty straightforward; otherwise, give any character not pulling the lever a 1-in-8 chance to be standing under the trapdoor.)
The ball bearings fill the room to approximately armpit level on a human; the effect is such that small creatures can freely run about on top of the ball bearings, especially if they have large or webbed feet, while larger creatures sink and have a lot of trouble moving. Humans and elves have their movement rate slowed to as if they were as heavily encumbered as possible and are -4 on all their rolls. Due to the elevation difference and the lack of mobility, they are also -4 to their AC (if any players complain about this affecting elves, tell them this isn't snow in the wilderness, it's steel balls in the underworld, so if they want any magical exceptions because they're elves they can have 1d6 damage a round for exposure to ferrous metal). Dwarves are covered up entirely, but can still move around as if heavily encumbered, but must save against whatever you think is appropriate to keep from inhaling a ball bearing. Hobbits, halflings, goblins, kobolds and other small creatures that are in the room when the trap is triggered manage to get to the top of the ball bearings if they roll at or under their Dexterity score. They can move about as normal, but must continue to move in order not to sink into the bearings. If any of these small creatures don't manage to make it to the top of the bearings, they begin to be slowly crushed, taking 1d4 damage a turn.
The room that the ball bearings fall from is also connected to the other goblin dwellings by tunnels (that are set at 45 degree angles along the vertical axis to prevent the bearings from escaping down the tunnels). When they hear the trap triggered (and, really, half of a megadungeon would hear this trap triggered), they rush through these tunnels and down into the room with the party floundering in the bearings. What they do from there is up to the ref, but it will probably involve extortion or killing.
Detection/Disarming: Observant PCs will notice both the chains, cables or gears attached to the doors (to make sure they close when the lever is pulled) and the trap door in the ceiling. Since the trap door opens downward, it would make sense that it is triggered by the lever. Something that wants to close all doors and release a trap door in the ceiling is probably a trap. Goblins clearly labeling a lever as a way to get treasure may or may not be a clear sign that it is a trap, depending on how you run goblins in your game.
The chains or cables or gears that secure the doors may be destroyed, rendering the ball bearings much less dangerous if a door is kept open and the ball bearings roll out into the rest of the dungeon (all characters should roll at or under their Dexterity at -2 when walking among ball bearings or fall, though). Other than that, the party's best course of action is probably to avoid the trap altogether.
Designer: This trap is the result of goblin ingenuity, whether as a community (if you run savvy, clever goblins) or an exceptional goblin leader (if you generally run dumb goblins). They take great pleasure in the way this trap gives them an advantage over larger creatures in melee combat. When they are done with whatever triggered the trap, they will bag all of the bearings, take the bags up into the tunnels that lead to the room above, re-set the lever and trap door and pour the bearings into the room to prepare for next time.