Haha, oh wow

- A square room with a small opening in the ceiling that allows light from sun/moon to come in.
- A chest full of identical saphires and some mirrors can be found in the room.
- One of the walls has carvings in which the saphires fit, but there are twice as many cavings as saphires.
- A closed stone double-door (which is what the player's will attempt to open)

The saphires have to be put in the right spots or, if put in the wrong ones, water will start to fall from the ceiling, and the door behind them will be closed. Now the tricky part is: the window in the celing is an Illusion with magical moonlight. So if they just keep trying they will actually drown.

The solution: use the mirrors to direct the moonlight to the wall oposing the carved wall. This will reveal markings that show where the saphires will go.