The tunnel descends into darkness.
Type: magical; Perception DC 28; Disable Device 28
Trigger location; Reset None
When the PC's get about halfway down the corridor, the entire area becomes bathed in bright light. Light Bomb(DC 19 Fortitude Save or become blinded for 1d4 rounds, those that make their save take a -2 penalty to vision based checks for one round); multiple targets (all targets within 60 ft)
There are several options for what comes after this trap. Of course, the obvious course of action is to use this trap as the opening gambit in a surprise attack. This can be used a few other ways, though. It can disguise another trap, help hide a secret door or just be used to diminish the advantage of a party with darkvision. It would also work well as a distraction for a fleeing NPC. As a last resort, you could just use this on its own, and watch the paranoia start.