The Trickle Chamber
Deep underground, our intrepid adventurers explore a network of caverns carved through bedrock by a long forgotten civilisation.
Upon entering one chamber, they see a large stone grid on the floor, roughly twenty feet by ten in size. There are two doors leading into this room: the one they entered through, and another leading out (sealed by a perfectly smooth stone slab).
Upon one square of the grid is a short stone pillar, one foot high. Affixed to the top of this is a shallow, carved stone basin, filled with an ethereal and intangible liquid which glows faintly in the darkness of the cavern. The basin has a hole in one side, and the liquid is trickling out, vanishing as it falls. The source is not depleted.
An adjacent square bears a similar (but empty) basin at ground level, with an inlet on one side which points in the opposite direction to the outlet of the raised basin. Neither basin can be lifted or rotated.
The other thirty squares of the grid are covered with stone tiles. Each of these had a strangely shaped carving: shallow channels with crisscross each tile.
If a character attempts to catch the mysterious liquid in their hands, it feels incredibly heavy but vanishes almost instantly on contact. It is immune to magical interaction.
Inspection of the tiles reveals that they are mounted on an ingenious gear system carved from marble. Each tile can be rotated through 360 degrees, given that all other tiles are currently aligned. They are quite heavy and require a strength score of 12 to move one. Each character can turn one tile per turn as an action.
As soon as a character rotates any of the tiles, a stone door falls into place, blocking the way back out. It is impenetrable and resistant to magic.
From this moment onwards, a swarm of 1d4 carnivorous bats enters the chamber every 30 seconds, through a small hole in the lofty stone ceiling.
Once a circuit of channels linking the raised basin to the lower one is completed, the glowing liquid flows along the path, draining from one bowl to the other. When the second bowl is filled, the weight of the magical liquid activates a hidden mechanism, raising the door which leads deeper into the caverns.