The floor is made of stone that flexes slightly when stepped on.
|The ceiling is filled with heavy barrels that fall on trespassers and crash through the floor into the underground lake below.
A DC 16 Wisdom (Perception) check detects a that one of the large stones that make up the floor is slightly raised above the rest.
A DC 16 Intelligence (Investigation) check shows that the stone floor is thin and flexes almost to the point of shearing when it is walked on.
When this stone is stepped upon, several barrels crash through the ceiling into the party and also crash through the floor into an underground lake 20 feet below. Creatures on the floor must make a DC 14 Dexterity saving throw or be hit by a barrel, which is open on the bottom and covers them. This also causes 1d6 hp of bludgeoning damage and traps the creature inside the barrel.
The barrel crashes through the floor, making a neat, barrel sized hole and plunges into the lake where it sinks to the bottom. After 1d4+1 rounds, the oxygen in the barrel will be exhausted, and creatures still in the barrel begin to suffocate.
Due to the suction, the barrels require a DC 18 Strength check to lift off and allow escape. Because of the cramped conditions in the barrel, no weapon can be used larger than a dagger or small hand axe. The barrel has AC 15 and 15 hp, and attacks from the inside are at disadvantage.
The stone pressure plate can be avoided with a DC 12 Dexterity (Acrobatics) check or jumped over with a DC 12 Strength (Athletics) check. It can be disabled with a DC 14 Dexterity check with thieves’ tools.