Here is another installment of exTRAPaganza, our Thursday over-the-top trap series.
This giant hall is the inside of a massive clock. There are gears everywhere along the walls, but nothing is moving. In one corner, a large wooden chest is jammed into a set of gears and levers. This was purposely done to stop the clock. If the chest is moved or opened, it slides away and the clock begins to run again.
The first thing that happens is a huge pendulum swings down in the center of the room. Everyone in the room must make a difficult Dexterity saving throw, or take bludgeoning damage equal to 2x (average party level) d6 damage. The pendulum then continues to swing every d4 rounds. The second time it comes through, the save may be made at advantage by spending a reaction.
After the clock starts, in 1d2 + 1 rounds, the hour gong sounds. It will sound d12 times, once per second. Each time the gong chimes, each creature inside the clock takes 1d4 sonic damage for every 4 levels. A Difficult Constitution saving throw cuts this damage in half.