Open the door labeled OBSCURA, and players are met with a bright flash of lights. Players with 15 ft of the door (or 1d4 players, if you're not tracking player position on a grid) are blinded for 1d20 minutes.
Furthermore, the room is a camera obscura and creates a magical duplicate of the players. These duplicates attack the players immediately. The duplicates are actually doppelgangers (refer to your favourite bestiary or monster manual for stats). Alternately, there are no doppelgangers, but some generic, low-level otyugh, derro, or your favourite dungeon critter.