The players already have the riddle when they find the room. Perhaps they heard it from a contact, or found it in an ancient manuscript; how they got it depends on your story. The riddle is as follows:
Six lights illuminate the day,
Faith in darkness saves it.
Consuming fire reveals the way,
Mutual destruction unlocks it.
The room is pitch black as the players enter (permanent darkness spell [Core pg 263] [SRD Link]). When the first candle is lit, or for any players who can see otherwise, see flavor text.
*There are six candles in the room (six lights illuminate the day).
*Each candle depicts an animal that is NOT a predator, with two exceptions: one shows a hawk, and another a snake. Lighting these two candles will open the path (consuming fire reveals the way, mutual destruction unlocks it).
*The path is actually a portal which opens in one of the walls to a section of the Shadow Plane. This section is a hallway which spirals outward before coming to a halt at the exiting portal. This portal should open to the player’s destination, wherever that may be. The hallway will seem long, and the players will be overwhelmed with a feeling of hopelessness when they enter. However continuing through it (faith in darkness saves it) will lead them out.
*When the players light two candles different from the solution, one of the two creatures rise up from behind each of the players and strikes at them. This functions like the spell “spiritual weapon,”[Core pg 348] [SRD Link] except that it is an illusion spell (and thus can be disbelieved) and does negative energy damage. Attack bonus is +3, damage is 1d8, Will DC 12 for 1/2.
*This works great as a secret entrance to a wizard’s tower or an old castle.
*It may also work as a last-ditch escape from a powerful enemy that has been stalking them. Perhaps they attacked a dragon in his lair, and found out they bit off more than they could chew? In this case, the riddle should probably be found in the room itself.
*Detect magic notes weak illusion magic and moderate conjuration.
*By increasing the “darkness” spell to “deeper darkness,” raising the resist DC of the illusions, and improving the attack and damage, this can easily be made into an encounter for any level.
This is a pretty easy riddle. I wouldn't recommend any treasure, unless the players have a difficult time with it; in that case, they could find the body of a dead adventurer with some masterwork gear that one of the players can use.