Friday Encounter: Chamber of Tremors

Here's a fairly simple dungeon puzzle I used to great effect at my last session. It's based on Zhang Heng's Han Dynasty-era seismoscope, which was used to detect the direction of approaching earthquakes. Growing up, I had a book on the ancient world with a diagram of the device, which astounded me then and stuck with me well into my adulthood - so it only seemed fitting it would one day find its way into my games.

For this encounter, there should be a large central room in a dungeon dominated by a strange sculpture - a four-sided pillar, with a dragon sculpted on each side, facing each cardinal direction. Each dragon carries a pearl in its mouth, and positioned directly below it is a frog with a wide open mouth, looking up. Close inspection will reveal that the pearls in the dragons' mouths are a separate piece, but cannot easily be removed by hand. There should also be four doors on the walls, leading in each of the cardinal directions.

In each of the rooms surrounding the central room, there should be some sort of interactive element that can generate a tremor. Examples might include striking a gong, dropping iron weights from a ledge, animating a heavy statue, or getting an ogre to strike the floor with its club. I'll leave the exact details up to you, in case you have other ideas - the important part is that the ground should shake on successful completion of each room's task. If the PCs find a way to create a tremor through means other than the intended solution, such as a thunderwave or similar spell, this should also count as a successful completion of the puzzle. The physics at play here is what's important, not the finer details.

When a tremor goes off in each room, it is felt by the mechanisms in the central room. Each dragon is attached to a lever connecting to a pendulum inside the pillar. When the ground shakes, the pendulum is shifted in the direction of the vibration, causing the corresponding dragon's mouth to open and the pearl to drop into the mouth of the frog below.

If the PCs remove the pearl from the frog's mouth, they will hear a clicking noise, indicating that a pressure plate inside the frog has been relieved. The pearls contain no special attributes. If they are placed back in the dragons' mouths, the mouths close and another vibration must be produced in that direction to release the pearl again.

Once all four pearls have been dropped in the mouths of the frogs, the dragons' and frogs' eyes begin to glow blue. If the party stands by the pillar when it is in this activated state, it lifts off the ground, serving as an elevator to take them to a previously inaccessible location in the dungeon.