The trapped area is quite large, and has a very high ceiling. Carvings on the wall depict storm clouds and lightning. Upon triggering, dark magical clouds appear in the corridor/room overhead, and begin dropping torrential rain and lightning on the characters. Anyone in the area has a 50% chance of being struck by lightning per minute, taking 4d6 HP in electrical damage (Save for 1/2 damage). The secret to disarming the trap is hidden in the carvings on the wall.
Optional: The exits close up, locking the characters within, so that they can't just run out of it.