Jaheira's Companion Quest - Baldur's Gate 2
Jaheira's Companion Quest only starts after you've completed both of the Bring the Poisoned Man to his Friends and Find Out What Happen With Montaron side quests. Unlike the other Companion Quests in this game it's worth noting that you will only unlock this side quest if you're currently romancing Jaheira... Or more specifically her LOVETALK variable is at least 27 (which means you've started the romance but aren't progressed far into it).
Note: If your LOVETALK variable is 27 after you witness Lucette attacking Xzar then Meronia will appear right away. Otherwise, Meronia will appear at some point in the future when you've had enough "love talk" conversations with Jaheira. There is another event that happens at LOVETALK 27 too with Jaheira involving a group of bandits that will take her hostage as they try to rob you. Sometimes this event will trigger immediately before this one.
When Meronia shows up and talks to your group she'll take Jaheira away with her. Another important tip is it's recommended you don't save while Jaheira is away from your party as this quest can break her romance. Rest one more time and Jaheira will return to your group - you'll next want to travel to the Galvarey Estate in the southwestern portion of the Docks District with Jaheira in your party.
When you arrive you'll be questioned by someone named Galvarey, it doesn't matter what you answer for this as it'll always lead to conflict. Galvarey and his posse drop quite a few magical items, make sure to grab everything (including the Reaching Ring) and then leave the Harper Hold.
At this point in time the quest will involve a lot of waiting around. You will need to wait a few days in game before you're approached by someone named Reviane. Reviane will also not appear until you've reached LOVETALK 35 with Jaheira. If you're wondering how you can raise LOVETALK with her, typically each conversation you have with Jaheira will raise it by 2. Check out my Jaheira Love Guide for more information about how this works.
Important: LOVETALK advances based on how much IRL time passes. The best way to advance your LOVETALK variable is to leave your game idle and check it once an hour. Usually 1hr is how long the timer is between each "Love talk" that happens in game. Each "Love Talk" advances the variable by 2 points. In other words, between some of these steps you'll have to wait a couple of hours IRL time.
If your Reputation is 16 or more and you choose the correct dialogue choices (1 then 2) you'll be able to avoid a fight with Reviane and her compadres. Otherwise you'll have to fight her and the Harpers that she brought with her. After encountering Reviane to advance this quest again you'll need to raise your LOVETALK to 41 and also wait a few days until you encounter Dermin.
Dermin is the next person who has been sent to deal with Jaheira and our group. When speaking to Dermin be careful what dialogue options you choose as they can result in Jaheira permanently leaving the party. Essentially you want to avoid threatening her or telling her that this whole event was a waste of time, that'll piss her off and make her leave.
Dermin Dialogue: For the first conversation choose options 1, 2 or 4 (3 will make Jaheira leave) and for the second dialogue prompt choose options 2 or 3.
Once your talk is finished with Dermin - Jaheira will have renounced her Harper status and you'll get an update in the Journal about it. To advance the quest again you're going to have to raise Jaheira's LOVETALK some more which will involve a lot of time passing. Since so much of this quest revolves around Jaheira's Romance and the LOVETALK variable I recommend you read my Jaheira Romance Guide which pertains specifically to this quest.
The TL;DR version of that guide is you're going to have to stand around a lot doing nothing to wait for Jaheira's LOVETALK to advance. Usually once an hour Jaheira will initiate dialogue with you and each time she does this her LOVETALK variable increases by 2. Between the first interaction with Dermin and the 2nd we need to raise our LOVETALK by 12 - this means at least 6hrs of waiting around.
Here is a different way of giving you the same information (also this is copy and pasted from the Jaheira Romance Guide):
Check LOVETALK Variable (mouse over Jaheira then enter it): C:GetGlobal("LOVETALK","LOCALS")
Reviane Appearance: LOVETALK 35 is needed and a few days must pass after the events at Harper Hold. The dialogue that precedes this meeting is, "I have been in a similar place as this before, though it was in the company of other Harpers. I will miss those times."
Dermin Appearance #1: LOVETALK 41 is needed and a few days must pass since you encountered Reviane. The dialogue that happens before Dermin appears is, "There is an ill wind in the air. Do you feel it? I feel a storm inside, in the distance."
Dermin Appearance #2: LOVETALK 53 is required and 6 days must pass since your last encounter with Dermin. The dialogue that happens when you're LOVETALK gets this high is, "How...how did that battle make you feel? Did you enjoy it?"
As aforementioned when your LOVETALK reaches 53 you'll be approached by Dermin a second time (pictured above). This time he tells you that a hit has been placed on Jaheira before leaving. You'll also get a journal update from this interaction with Dermin titled, The return to Harper Hold.
The next time you rest in an outside area you'll wake up to find that Jaheira is gone. A man named Terminsel will approach your group and give you a note from Jaheira that will explain what happened. Essentially, she returned to the Harper Hold to try and settle things amicably.
You can probably guess our next destination now - to Harper Hold! Return to the Galvarey Estate in the southwestern portion of the Docks District and you'll find Jaheira on the 2nd floor. Speak with her and she'll rejoin your party after which you'll have to fight your way back out through the 1st floor.
Once you're outside everything returns to normal - at least for the next 2 days ingame. When 2 days has passed and your LOVETALK increases to 55 you'll encounter Dermin a third time when you sleep in an outside area. This time a confrontation with Dermin is completely unavoidable. Dermin brings some company with him this time, he'll have 4 other Harpers that will assist him in the fight.
Dermin Appearance #2: LOVETALK 55 is required and 2 days must pass since you raided Harper Hold for Jaheira. The dialogue that precedes this event is "Ah, the dawning of our day, though it is very rarely that we match our sleep to the cycle of day or night."
After defeating Dermin you'll be in the final stretch of this quest and also the part that most people cheat on. When you encounter Dermin for the last time and fight him your LOVETALK will increase to 57. To "complete" this quest you'll need to raise Jaheira's LOVETALK to 70 (at minimum). This means at least 6 - 7 hours of standing around waiting for Jaheira's dialogue to trigger from this point on.
If you'd like to cheat and spawn Terminsel right now then use the following 2 Console Commands:
C:SetGlobal("TerminselSpawn","GLOBAL",5)
C:CreateCreature("jatermin")
When Terminsel shows up he will ask Jaheira a single question after which your group will be rewarded 100k EXP and a Harper Pin (assuming you meet the other requirements). Below is a quick list of all the variations that can happen depending on your Reputation and whether or not you've failed the romance with Jaheira.
Reputation 10+ & Romancing Jaheira: 100k EXP & Harper Pin
Reputation <10 & Romancing Jaheira: 100k EXP
If not Romancing Jaheira & 10+ Reputation: Harper Pin
No reward if you're not romancing her and have a Reputation below 10.