Hidden within the Lower City Sewers lies the Guildhall, which is led by Nine-Fingers Keene, and upon speaking with her, you learn that she needs your help.
It turns out that she knows Jaheira and one of her comrades (Minsc) has started to cause trouble and is now a target of The Guild.
Nine-Fingers Keene asks you to put a stop to Minsc in a violent manner, but Jaheira will have other plans and it is up to you to decide how you will solve this issue.
Aid The Underduke Quest Walkthrough
After speaking with Nine-Fingers Keene, you will need to make your way to The Counting House and get into the lower floors where the vaults are.
This can be done by speaking to Head Clerk Meadhoney, who appears to be the third teller at The Counting House and he appears to be nervous.
Passing a series of Ability Checks will allow you to get a Bank Pass that will allow you to make your way to the vaults. (Meadhoney is nervous about the Stone Lord’s visit)
There are other ways to get a Bank Pass to open an account, which costs an outrageous amount of Gold (10,000) or you can always sneak your way in.
If Jaheira is in your party, there are additional options for you to choose from when speaking to Head Clerk Meadhoney that require lower rolls. (DC 10).
Progressing to the end and making your way to the high security vaults will cause a cutscene to begin where The Guild’s trap has failed. (This was planned earlier)
You end up fighting Minsc’s minions as he and a fake Jaheira leave, which gives you the chance to also save a certain Rakath Glitterbeard, who can give you the Return Rakath’s Gold quest.
After you have gotten out of The Counting House, you will need to chase after Minsc and the fake Jaheira, who will be in the Abandoned Cistern.
You can reach the Abandoned Cistern by heading to the Lower City Sewers and making your way to the western part of the map.
Passing through a Sluice Gate will allow you to cross paths with Minsc and the fake Jaheira again, where a fight will happen.
You can decide on taking down Minsc with Non-Lethal attacks if you wish to save him later and this will allow you to progress with the Aid The Underduke and other quests.
After dealing with Minsc (Either killing or saving him), you will need to head back to the Guildhall and find that the Zhentarim are about to attack The Guild.
You can choose to side with the Zhentarim but helping The Guild seems like the better option and the fight will start after you approach Nine-Fingers Keene and a cutscene happens.
Alongside the members of The Guild, you will have to defeat all of the Zhentarim in the area and leave none alive.
Once the fight is over, whoever you sided with will be in control of the Guildhall and you will need to speak to them in Nine-Fingers Keene’s office.
This results in you being thanked for your help and the assurance of assistance when the time comes to fight the Absolute.
Aid The Underduke Rewards
Completing the Aid The Underduke will result in you gaining new allies as part of the Gather Your Allies quest.
This will lead to you being assisted by either The Guild or the Zhentarim depending on who you sided with during the quest.
Verdict
While The High Harper and Return Rakath’s Gold quest are intertwined with Aid The Underduke, it is most likely that these will all be close to the same time of completion.
Siding with The Guild is preferable to most players as the fight is easier and this seems like the more heroic choice in the game.
During the final fight, the members of The Guild are capable of defeating the Zhentarim and the addition of your parting fighting alongside them makes the battle end sooner.