Toxic Ceiling

The ceiling is made of cloth. A door leads out.

Mechanical Trap

The top of the door has a blade on it that cuts the fabric, which dumps toxic sand stored in the ceiling when the fabric is torn. A DC 8 Wisdom (Perception) check detects that the top of the door is nearly flush with the ceiling. A DC 14 Intelligence (Investigation) check reveals that the cloth appears to be under load or stress. When the door is opened, the blade springs out from the top of the door and tears the fabric. All targets in the room must make a DC 12 Dexterity check or take 1d6 bludgeoning damage from the falling sand. Additonally, all target in the room must make a DC 14 Constitution saving throw or becomes poisoned for 1 hour. Targets that took damage from the sand are at disadvantage on the Constitution saving throw. If the door is examined, a DC 14 Intelligence (Investigation) check reveals that the knob has a complex locking mechanism that can be defeated with either a DC 14 Dexterity check with thives’ tools, or a DC 16 Intelligence (Investigation) check. Failing either or both has not effect, but the blade still cuts the fabric if the door is opened.