Suspended above a 10-foot-square of thin stone ceiling is an acid-filled container. Any weight of more than 20 pounds placed on a hidden pressure plate in the floor beneath this area triggers the trap. The pressure plate can be spotted with a DC 15 Wisdom (Perception) check. A character studying the area can determine the pressure plate is a slightly different color than the rest of the floor with a DC 15 Intelligence (Investigation) check. Wedging an iron spike or other object under the pressure plate prevents the trap from activating.
When the trap is triggered the container holding the acid opens, pouring it onto the ceiling. Immediately stone and acid rain down upon any creatures standing beneath. Creatures in the area must make a DC 15 Dexterity saving throw. Creatures who fail take 11 (2d10) acid and 11 (2d10) bludgeoning damage. Creatures who succeed take half damage.