This quest starts when you find Mrs. Argansa weeping, and Langolin praying beside her. Talk to them and you will find out that her husband is being held prisoner by Draak's minions, and Langolin says he can resue him if you lend him the Crystal of Wisdom and 3 potions of immortality. Though it may not be a good idea to trust just anyone you run into with a crystal.
Go down to level 17, 18 and 19, all of which will contain Fountain of Immortality, each will give you a Potion of Immortality and a potion of youth. Now continue down to level 20 where you will find a set of cells, one containing the body of Mr. Agansa. Search the body and you will find a scroll, telling you not to trust Langolin, he was only trying to get the Crystal of Wisdom from you, and the potions of immortality were only an excuse to make it seem more believeable.
Elsewhere on that level you will also find Langolin, blocking the entrance to a room filled with scorpions and a crystal, talk to Langolin and he will attack you (be careful, his first hit can hurt quite a lot). After you have defeated him, and the scorpions behind him, you will find that you are unable to remove the crystal from the pedestal. Remeber how Langolin wanted the Crystal of Wisdom? Well, he wanted it to remove this crystal. So go and get the Crystal of Wisdom and use it to click on the other crystal, which will cause the Crystal of Compassion to fall to the floor where it can be picked up.