When you enter Marlowe's home and talk to him, he will say he is a merchant from Calimshan. He will also mention that a lich named Vongoethe has stolen his daughter's soul and that he is hiding in the Cave of the Dead.
Now, you need to go to the entrance to the cave and talk to the monk. He will let you in. When you enter, don't attack the lich straight away and listen to what he has to say. Vongoethe will say that the merchant Marlowe and him had an agreement - Marlowe will exchange his own soul in exchange for him having 20 years of success as a merchant. The lich has simply taken Malla's (Marlowe's daughter) soul after Marlowe refused to hold up his side of the bargain and sacrifice his. Promise to return with Marlowe and head back to Marlowe's house.
Confront Marlowe with what Vongoethe said and Marlowe will admit. Everybody will then return to the lich. Marlowe offers to let go of his soul and the lich will give you Malla's Soul Stone.
Now you have three choices. You can either:
- Let Marlowe die, go back to Malla and restore her with the stone. For this you will gain 10,000 XP and a reputation point if you give her 1000 gp.
- Save Marlowe and kill the lich. If you choose this option Vongoethe will summon several skeletons, Skeleton Archers, and Banshees which can cast Wail of the Banshee. After the lich is dead you can give Marlowe the soul stone for 15,000 XP and a Reputation point.
- You can also let Marlowe die and keep Malla's Soul Stone, as you can use it as an ioun stone. Wearing the soul stone will give you an extra point of Constitution and Dexterity, and you will also be able to cast Neutralize Poison once per day. Only neutral- and evil-aligned characters can wear it.