This trap activates similarly to a pit trap, but the pit is actually a metal coffin concealed under the floor that drops into a body of water under the floor once a creature falls inside. The pitlike opening is 5 feet across, leading into a hollow metal coffin that’s 10 feet deep. A creature that steps onto the opening and fails a DC 20 Reflex save falls in, taking 1d6 points of falling damage. The top of the metal box then automatically shuts and locks (Disable Device DC 30, accessible from either side), and the fastenings holding the box to the floor release, causing it and anyone still inside or standing on top of it to plummet into a body of water, typically either fast-flowing or at least 200 feet deep. The box sinks at a rate of 60 feet per round, and could be carried by the current. The box is airtight, and contains enough air for a single Medium creature to breathe for 10 minutes before being at risk of suffocation. If the box is successfully opened while underwater, the water takes 1 round to fill it, during which time any creature inside the box must succeed at a DC 15 Strength check to escape the box against the power of the rushing water.
Pathfinder Roleplaying Game Horror Adventures © 2016, Paizo Inc.; Authors: John Bennett, Clinton J. Boomer, Logan Bonner, Robert Brookes, Jason Bulmahn, Ross Byers, Jim Groves, Steven Helt, Thurston Hillman, Eric Hindley, Brandon Hodge, Mikko Kallio, Jason Nelson, Tom Phillips, Stephen Radney-MacFarland, Alistair Rigg, Alex Riggs, David N. Ross, F. Wesley Schneider, David Schwartz, Mark Seifter, and Linda Zayas-Palmer.