|A metal rod extends out 2 inches from the floor, and holds the door open. The door opens out. On the other side of the door, powerful springs are present. A pressure plate in the room causes the rod to descend into the floor, closing the door. The room is filled with an unbreathable gas.
A DC 12 Wisdom (Perception) check shows that the rod can retract, and a separate DC 12 Wisdom (Perception) check reveals the springs behind the door. The pressure plate can be spotted with a successful DC 14 Wisdom (Perception) check. The air in the room can be detected as not breathable with a DC 14 Intelligence (Nature) check from just outside the room. This roll has advantage if checked from within the room, and no check is necessary once the door shuts.
When the plate is activated, the door swings shut. Those within 5′ of the door may either try to escape via a DC 14 Dexterity check, or stop the door from closing. A successful DC 12 Dexterity check followed by a DC 12 Strength check will keep the door from closing. However, as soon as the door is left unattended, it will shut. Once shut, it requires a DC 16 Strength check to open the door. The door can also be broken, and has AC 15 and 27hp.
Those in the room once the door shuts will begin to suffocate after 1 round.
The pressure plate can be disabled with a DC 14 Dexterity check, and the check has advantage of thieves’ tools are used. The springs on the back of the door can also be disabled with the same check, or broken. A DC 16 Strength check is required to remove them from the door. They can also be destroyed, and have AC 19 and 10hp.
Once the door is shut, it must be opened or destroyed. The plate has no effect, and the springs cannot be accessed.