A spiral stone stairway leads upwards.
|The stairway compacts as it is travelled, eventually turning into two flat grist stones.
A DC 12 Intelligence (Investigation) check reveals that there is wear on the outer facings of each step. Those with stonecutting may add 2x their proficiency bonus.
When a target gets about halfway up, the stairs begin to compress, causing the floor to raise and the ceiling to lower. After 1 round, the exits at the top and bottom are inaccessible, and 2 rounds later, the floor and ceiling meet.
Those on the stairway may attempt to move to the top or bottom exit. Due to the decreasing size, they must be able to move to the exit and have an action to spend to exit the stairway. It requires a DC 12 Dexterity (Acrobatics) check to successfully escape. Creatures of size large or more have disadvantage on this check.
Those still on the stairs take 3d6 bludgeoning damage when the floor and ceiling meet. The stones continue to turn for 1d4 rounds. Each round the targets take an additional 3d6 bludgeoning damage.
After the turning stops, the stones take 2 rounds to return to normal.
The top or bottom of the stairs can be disabled by using spikes or something similar to wedge in place. This requires a DC 12 Strength check.