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Ferg Drogher
Involvement
Though posing as an unscrupulous trader on the surface, Ferg is actually a Sharran lookout standing watch for Shadowheart's return. The party's interactions with him are entirely dependent on whether Shadowheart is in the party at the time, and whether or not she rejected Shar in The Chosen of Shar.
If Shadowheart is in the party, he will beckon to the party when they come close. If she rejected Shar, he will threaten her and direct her to the House of Grief to receive punishment. When this dialogue finishes, he walks away forever, locking the party out of his trade goods.
If Shadowheart embraced Shar, he will remark that he's been waiting for her return and provide the same directions to the House of Grief. He hints at brewing unrest within the cloister, foreshadowing the upcoming fight with the Mother Superior.
If Shadowheart was given the 
Noblestalk, then she may ask about the child she saw in her memories, Rennald. Ferg will reply that Rennald is no more, and that he will leave the rest for Shadowheart to figure out.
Special stock
Ferg has a special stock of goods, which he can only be convinced to sell with a Persuasion check if Shadowheart is not in the party or if she has not rejected Shar. If Shadowheart embraced Shar, having her in the party will bypass this check. The party can also bypass this check by using 
on the corpse of the refugee on the cliffs of northeastern Rivington and learning about the 
Gemless Ring.
Notes
Although Ferg Drogher can be a fully functional merchant, he is not marked as one on the minimap. That may be a bug or it is intended because the ability to trade with him depends on certain story choices to be made before.
