You met a merchant named Barcott in Minallah. He is trapped in town because several rogue thahds have taken up residence in his workshop. He asked for your help with their removal. His workshop is in the Promenade.
Getting to the workshop is easy. Once you enter the Promenade by the west entrance, just follow the south path all the way to the east end and you will find a door, just south of where you killed the Shrouded Worm. This is the workshop.
The thahds will not attack you when you enter. You will need to speak to Krog, who is the in the center of the room. With a Leadership of 5 you can convince him to leave and earn reputation with the Rebels. Otherwise you can kill him (or tell him ahead of time you will for Shaper reputation) to complete the quest.