For this quest you must interrogate Mydon the Elder, a silent priest who cut out his own tongue. You will find him in the guesthouse of the Valley of Dreams. The guesthouse is guarded, so you will need to sneak in from the roof or kill the guards first. Either way, you should enter the guesthouse through the marked door to trigger a cut scene, during which you will have an opportunity to question Mydos. When you have learnt everything that you need to know, the quest will complete.
Note: If you chose not to kill the guards when you infiltrated the bathhouse, they will be waiting for you when you leave the room. Be prepared to run or fight them; they are relatively weak.