Irresistable Offer

From the introduction:

This short adventure is designed for four characters of 8th level. It is generic enough that it could be inserted in to any campaign setting. The adventure assumes that the PCs have made an extended foray deep into the earth at some time. They awake in the morning to find one of their companions (or merely an NPC friend) was abducted during the night. To seal his return they find themselves pressed (or hired) into service by someone known only as “the Mind”. Only after they have found and destroyed a rival merchant house run by an Aboleth Overlord and his subjugated minions do the heroes have their friend returned to them.