This is an entry for a contest – tell us about one of your traps…!
On first glance, this room seems to just be a room with a sandy floor. Digging down shows that there is packed sand down to about 12 feet (this takes about 2 turns). The door opposite has a tarnished brass pull handle with a lock. The door is unlocked.
Opening the opposite door using the handle, opens a valve that allows water to quickly seep into the sand. If the party exits fairly quickly, they probably don’t notice (Hard Perception check to notice). If they dawdle, anyone in the rear of the party will start to sink. After a few minutes, the room is basically quicksand. The party will have to deal with this on the way out or find an alternate exit.
Just pushing the door (when unlocked) or breaking it down will bypass the trap. When on alert, the cultists lock this door; any attempts to test the door sets off the trap.