Lava Flood CR 14
Four rounds after a person enters the room, the doors deactivate (or close and lock) and a portion of the ceiling melts, allowing lava to surge into the chamber at a horrific speed. The lava covers half of the room to a depth of one foot in the first round, then the entire floor of the room on the second round. Each round thereafter, the lava depth increases by one foot until the room is completely flooded with lava 11 rounds after the trap is triggered.
All creatures in the lava take 20d6 points of fire damage per round of immersion. After an hour, the lava reverses its flow, draining back into the outer rivers over the course of one minute and then resealing to reset the trap.
Type magic; Perception DC 30; Disable Device DC 35
Trigger visual (true seeing); Reset automatic (1 hour)
Effect lava floods the room; never miss; Onset Delay (4 rounds); multiple targets (everyone in room); 20d6 fire damage per round plus suffocation (see above).