I’m planning to inflict this room on my players 3 times over the course of them gaining several levels. The room will look identical each time, but be subtly different. This is the first version of the room.
The room is 35ft. square with the walls and floor made of masonry. There is a 5ft. square stone platform in the middle of the room raised about 6 inches from the floor. A large wooden chest sits in the middle of the raised platform. The room has a high ceiling of about 23ft. There is only one door in this room.
The platform is, in fact, a pressure plate and any change in weight on the platform will trigger the trap. DC20 to find the trap trigger. Disarming the trap is a different matter. The party would have to figure out that the block above the doorway is the “trap”, and have to find some way to prevent it from falling (DC25 to disarm trap).
If the trap is sprung, a huge block of stone will drop to seal the door. The block of stone is very heavy. It’s 5ft. square and 15ft. tall, so it will be almost impossible to lift. (Google tells me that 5*5*15 feet of sandstone should weight ~56,000lbs.) The block is a tight fit. As the block falls, it releases a massive flow of water into the room from a cistern above which the block was holding back. The water will quickly fill the room at a rate of 5ft deep per turn. The room will be full in 5 turns. The room holds 28,175 cubic feet of water.
The cistern above is a chamber 20 feet square and 71 feet deep, holding 28,400 cubic feet of water. Enough to fill the room below with a little bit of water leftover filling most of the aqueduct connecting the cistern to the trapped room. (I was nice to the party when they were in here. My cistern filled by a natural spring in another chamber above the cistern that the players were able to climb up to and escape.)
Let the party be creative.
Air will run out rapidly in the room. How long can the party hold their breath?
(It turns out 5e D&D swimming speed combined with suffocation rules make it very easy for the party to escape this trap. They can hold their breath 1 minute plus 1 minute per CON bonus. Swim speed is half normal movement speed. At maximum it would take them 5 turns to swim out of here once the water stopped flowing. 5 turns is only half a minute in D&D, so even without and CON bonuses, they can escape in 30 seconds, with 30 seconds to spare before they start drowning).
It’s up to the DM how sadistic they want to be. Electric eels in the water could be fun 😀
Perhaps once the water stops flowing when the room is full, they can swim out the way the water came in and escape via the now empty cistern?