When triggered, any exits seal up, and the walls (made of metal or stone, of course) begin moving in. Characters have 5 minutes to either jam, disarm, or deactivate the trap.
Jamming might be possible with metal spikes, which take one minute each to set (though multiple characters may do so at the same time); after setting at least four spikes on one side, roll 1d100: 01-50% the wall is still coming at the same rate; 51-85% the wall is slowed, adding 2d4 minutes before being crushed; 86-00%, the wall is halted. For each additional spike after the initial four, roll again, and add 05% to the dice roll. Spikes may continue to be set until the characters are crushed. The walls will try and meet in the middle until at least one of them reaches that point, at which time the walls will retract, the trap will reset, and the exits will open again.
Optional: Certain spells and/or magic items may defeat or disarm the trap.
Optional: There may be a puzzle component to the trap, which, if deciphered in time, will disarm it.