Look...
This is where the riddle is from... and this is the answer...
Here is the text of the riddle that Clavicus Vile asks to Cyrus.
The context
Clavicus Vile is sitting in a chair with a dog at his right side in front of a little house in a green meadow. Cyrus has offered Clavicus Vile to play a game because Vile said he was bored. But Cyrus had to wager his soul against Iszara's soul in the contest. Now Vile has to choose a riddle and Cyrus has to answer right to free Iszara's soul.
Vile:
And now that I have your complete attention. You're ready for your test. But, before we start...
Cyrus, did you have a classical education?
Cyrus says nothing
Vile:
Good, good, just checking. Just that a thinking man, a well-educated man might have an unfair advantage in this test. But no! You're a hero...a man of action, right?
Cyrus:
You're talking to yourself Vile. I'm ready. Let's go.
Vile, the riddle:
So you probably won't recognize this old chestnut, taken straight from Paulus's "The Stars on Their Shoulders". Ready? Listen carefully! I won't repeat it!
Two doors. One leads to the loss of your soul, the other leads to the return of Iszara's.
* Two guardians. One always lies, one always tells the truth.
* You can ask one of them just one question. Just one!
The game begins... Choose wisely... Hee hee!
Cyrus, possible questions:
1. Which door is corect?
2. Which door is not corect?
3. Which guard is lying?
4. Are you the guard who lies?
5. Is he the guard who lies?
6. Is Clavicus Vile lying?
7. Which door would Vile say is correct?
8. Which door would the dog say is correct?
9. Which door would the other say is correct?
10. Do both doors lead to death?
11. What is behind this door?
12. What is behind the other door?
The only question that enables to guess the good door in all cases is:
9 - Which door would the other say is correct?
There are four cases but in all, the guard will indicate the wrong door, and Cyrus will have to choose the other one.
* if Cyrus asks the liar, this one knows that the other guard would say the good door, but as himself is a liar, he'll indicate the opposite door, the wrong door.
* if Cyrus asks the trusty one, that one knows that the other guard is a liar and would say the wrong door, so he'll report truely the lie of the other guard. In both cases the reported door is the wrong one, so Cyrus just has to choose the opposite door than the one reported.
* if Cyrus is answered THE RIGHT DOOR, he has to choose the LEFT DOOR
* if Cyrus is answered THE LEFT DOOR, he has to choose the RIGHT DOOR
For all the other questions, both of the doors are wrong. Obviously the game considers the other questions as non fitting ones and removes the 50% chance that one should usually have in choosing at random. So, that if the gamer asks another question than the #9, he has 100% chances to pass through the wrong door.
SOURCE -
http://til.gamingsource.net/riddles.shtml