Continue your search by heading across the Skunk River (travel south from the "A Ford Across the Skunk River" area), then turn southwest after you cross the river. Here you should be subjected to a not-so-random encounter, stumbling upon the scene of a merchant and his guards being attacked by some Trolls. Cut down the first Troll you spot and let the Merchant's Guard occupy a second Troll and once one of the Trolls dies, a Branded Troll will appear to the west and attack you. This giant is a stiffer challenge than normal Trolls are, having a much higher Attack bonus, more HP and slightly higher Armor Class. More worryingly, they're not deterred by fire, so be sure to use acid damage - either Trollreaper or a humble Acid Splash cantrip will work.
This curious trait will be mentioned by the merchant, Dalton, who will strike up a conversation with you after the Trolls are dispatched. He'll also bring up the shoddy ruler of these lands who can't seem to keep the roads clear. Pick whichever moral option you wish, as either way the merchant ends up mentioning a nearby mage named... you guessed it, Bartholomew. According to Dalton, Bartholomew is an "expert on trolls", which is certainly a useful expertise to have at this present moment.
Ask whatever questions you will about this Bartholomew to learn a bit about him, then ask "Any other dangers on the road?" to learn of a Hodag Lair, which he'll mark on your map. This will please Amiri, who has been searching for another conquest after Tuskgutter. If you want to coerce Dalton into sticking around in your lands and helping the local economy, pick the dialogue option "I can see you're an experienced trader. Would you consider operating exclusively in my barony?". He's clearly not a fan of standing in your... rather under-developed lands, but succeeding at a [Diplomacy 15] or [Intimidate 15] check will convince him otherwise, with the former being a [Lawful Good] action and the latter being [Neutral Evil].
|Reward: For convincing Dalton to trade in your lands|
Finally, if you're truly an evil wretch you can try to hold Dalton up [Neutral Evil], or evil kill him and steal his wares [Chaotic Evil]. In the latter case, you'll have to fight Dalton and his guard, who are tough, but shouldn't be any match for you. This is inadvisable, however, as all you'll get are three Potions of Cure Light Wounds, 100 GP and a Wand of Acid Splash from Dalton, which he planned to give you (sans the wand) for saving him from the trolls. You're probably better off just getting him to do business in your lands and parting on those terms. This doesn't preclude you from looting his wares, especially the chest by his wagon, which contains a Token of the Dryad.
When you're done with Bartholomew - and his loot - continue on to the Lone House area, which you can reach by following these directions from the "A Ford Across the Skunk River" area:
South (cross the Skunk River) Southwest South
When you arrive at the Lone House area, make your way to Bartholomew's house, which is in the northeastern corner of the area. If this is your first visit, consult the page Lone House for information about the area, including encounters, traps, treasure... all that good stuff.
Talk to Bartholomew (assuming you're done doing everything else in this area) and ask him about the "local trolls who are resistant to fire?", to which he'll seem surprised before immediately mentioning that just such a troll was caught in one of its fire traps. Perhaps studying its corpse will give Bartholomew some insights? The only problem is, you'll need to make it a corpse, first, as the mage's traps are clearly not suited for the task. Assuming you're ready, pick the option [Accompany Bartholomew] "Lead the way.", then kill the Branded Troll when you arrive.
After this easy fight, Bartholomew will use his magic to locate a hammer-encircled-by-thorned-vines on the beast's chest. Too bad you can't get one of those, eh? A brand seems a small price to pay for fire immunity. Bartholomew will excise the brand, then hand you a key to his laboratory, instructing you to go there while he does... whatever he's going to do. Loot the Branded Troll when Bartholomew leaves, then head north to reach the mage's laboratory, the entrance to which is disguised as a humble barn.
Enter the laboratory, head down a ramp and note a secret door [Perception 15] along the wall to the northeast. Open the door (provided you find it, of course) to find a trapped [Perception 22] check inside, which you should carefully disarm [Trickery 22], unlock [Trickery 20] then loot. Your prizes include a Cloak of Resistance +1, Bracers of Archery, two Scrolls of Fireball and 371 GP, among other, less interesting treasures.
Leave the room, pass through a gate to the southeast, then turn northeast to reach a proper dungeon. This Bartholomew character might be a little more morally dubious than he originally let on. Continue northeast and you'll be pestered by a troll named "Dimwit", who croaks variants of "Bor-ba" at you from behind the bars of its cage. Why did Bartholomew send you here, knowing you'd find evidence of his cruel experiments? You'll get a chance to question him soon enough, as he'll arrive and tell you what he discovered about the branded troll.
|Reward: For learning about the troll's fire immunity|
Apparently these fire-resistant trolls are being mass-produced by some artifact. What artifact, and who is using it? Questions Bartholomew can't answer, unfortunately, but some answers are better than none, and if you can cut the head off this particular serpent, perhaps the body will wither? In any event, Bartholomew rewards you with a Wand of Acid Arrow, just in case you don't have enough ways to rid yourself of trolls yet, after which you'll get the chance to confront him about his dungeon.
The crux of the... misunderstanding seems to be that Bartholomew doesn't view trolls as sentient beings. Your character's view on the matter (and perhaps the issue of slavery) can serve as a fine guide for your actions here, or failing that, just following the alignment values and the whims of your companions. Picking the [Chaotic Good] or [Chaotic Neutral] responses will force Bartholomew to release or kill the troll, respectively, but it'll also anger him. On the other hand, the [Lawful Neutral], [Neutral] and [Lawful Evil] responses will allow Bartholomew to keep his subject.
If you have Octavia in your party, she'll object at any outcome that keeps the troll alive and behind bars, forcing you to pick a [Neutral Evil] action to keep Bartholomew appeased, otherwise leaving [Chaotic Good] or [Neutral] options that side with Octavia and anger Bartholomew. If you side with Bartholomew, Octavia will not hold a lingering grudge against you, but Bartholomew is another matter entirely... Aside from leaving Octavia behind before exploring Bartholomew's laboratory, there's no way to avoid this conflict and keep both sides happy.
Whatever decision you make, continue northeast and loot a chest on a shelf to score a Token of the Dryad, then pick two books off some bookshelves, including "Temple of the Roses" by Elestin of Sinaria and Bartholomew's Diary.
Time to leave this area and get on with the search. You have have learned something of the troll's nature, but you haven't gotten any closer to finding out where they are, and who is behind their artifact-induce immunity. Next stop, the Ruined Watchtower.