The De'Arnise Keep Has Been Invaded - Baldur's Gate 2 Side Quest
This quest begins by speaking with Nalia inside of the Copper Coronet in Athkatla. Nalia will approach your group and ask them to help her and her family against a siege at their home, De'Arnise Keep. Accepting this quest will add De'Arnise Keep to your world map and allow you to travel there. I strongly recommend you pick up this quest and complete it early on as it'll net you a few really good items.
I recommend you buy the Sword of Flame +1 from the Adventurer's Mart in Waukeen's Promenade before coming to this dungeon. This weapon will allow you to easily defeat trolls once they fall to the ground because you need to deal either Acid or Fire damage to permanently kill them. Having this weapon will make exploring this dungeon significantly easier.
When you are ready to begin this quest fast travel to De'Arnise Keep and visit the small encampment in the southwestern portion of the map. Here you'll meet Captain Arat who will give you some information about what to expect inside of the keep. More importantly, just north of Captain Arat is where you'll find the secret entrance allowing you inside.
If you'd like to see detailed maps for De'Arnise Keep I recommend you check out my De'Arnise Keep World Overview page. That has every floor of the keep mapped out along with details about every notable thing you can find within the keep. All you need to do to complete this quest & dungeon is defeat TorGal in the basement; however I recommend you spend a little bit more time completing all of the side objectives here as there is good loot and EXP to be had.
Important: One thing I should mention immediately is that once you finish this quest and defeat TorGal the moment you exit the keep it will "reset" and any loot inside that you didn't grab will be lost forever. Keep this in mind before you rush through the dungeon and miss out on some of the best items!
The first thing you will want to do inside the keep is visit the 2nd floor and obtain the Keep Key which is required to open many of the locked doors on the 2nd floor. With this item you'll be able to reach 2 of the Flail Heads that we need to make Flail of the Ages +3 as well as the other 3 important magical items, The Kneecapper +1, Elven Court Bow +3 and Frostreaver +3.
In an effort to make exploring this dungeon easier, I have included a check list of the most important things you'll want to do here.
Checklist of things to do
1. Collect all 3 Flail Heads and craft Flail of the Ages +3
2. Collect The Kneecapper +1, Elven Court Bow +3 and Frostreaver +3 from the treasure room
3. Defeat the dogs in the courtyard and make Dog Stew in the Kitchen for 11,500 EXP
4. Lower the drawbridge in the courtyard and earn 29,750 EXP
5. Feed the Umber Hulks the Dog Stew you made for 18,750 EXP (then kill them for another 20k EXP)
Fighting the Umber Hulks in the basement is going to be the most challenging part of this dungeon as they're likely going to be a lot higher level than you are. I'd recommend using the Dog Stew to skip them if you're low level -- alternatively you can do both use the Dog Stew to skip and fight them for double EXP. If you keep wiping to the Umber Hulks and you don't know how to beat them I recommend you check out my Baldur's Gate 2 Beginner's Guide for more information about how to get gud at the game.
To feed the Dog Stew to the Umber Hulks you'll have to use Invisibility on a character to sneak past them and then place the Dog Stew in the tunnel in the southern portion of the dungeon. Below is an overview map of the dungeon showing you where to put the stew and where you can find TorGal.
After you've collected all of the loot you want from this dungeon it'll be time for your showdown against TorGal. Make sure that you're ready to complete this dungeon before engaging him - as aforementioned once you're finished with this quest all of the loot inside this dungeon will despawn. Defeat TorGal then when you're ready to complete this quest exit De'Arnise Keep.
If Nalia is in your party once you exit De'Arnise Keep you'll complete the quest and earn 45,500 EXP as well as up to 10,650 gold if you choose the right dialogue options. If Nalia is dead or not in your party then you'll have to visit the wooden palisade in the southwest portion of De'Arnise Keep and speak with Captain Arat here.