This guide will walk you through the Mage Stronghold quests in Baldur’s Gate II.
Overview
To claim the mage stronghold, you’ll first have to talk to Tolgerias the Cowled Wizard in the Council of Six building in the Government District. Tolgerias will ask you to track down a murderer named 
Valygar Corthala. This will cause Valygar to show up in his cabin in Umar Hills. When you talk to him there, you’ll learn that the Cowled Wizards only want him so they can use him — or maybe just his blood — to enter the Planar Sphere in the Slums District. By adding Valygar to your party, or by killing him and carrying his body with you, you’ll be able to enter the Planar Sphere yourself.
During your time in the Planar Sphere, you’ll meet some Solamnic knights, you’ll confront Valygar‘s ancestor Lavok, and you’ll kill Tolgerias. When you eventually escape from the sphere, Lavok will bequeath it to you (assuming that you’re playing a mage or a sorcerer). For more information about these events, please refer to the Planar Sphere page.
When you return to the sphere after taking ownership of it, the Solamnic knights will ask you to find a way for them to go home. Right after this, a Cowled Wizard named Teos will teleport into the sphere. He’ll offer you a deal: the Cowled Wizards will forget that you killed Tolgerias, and they’ll allow you to continue using the sphere, and all you’ll have to do is complete some quests for them. No matter how you respond, you’ll accept this deal. This will lead you to receiving two quests from Teos, plus the quest from the knights. These quests are described below.
Quest 1: The Solamnic Knights
After taking ownership of the sphere, the Solamnic knights stuck inside will ask you to find a way for them to go home. With the sphere permanently grounded in the Slums District, you won’t be able to use it for the job. For some other ideas, you should talk to Teos while he’s in the sphere. Teos will suggest that you meet with Ribald Barterman in the Adventurer’s Mart in Waukeen’s Promenade or one of the paladins in the High Hall of the Radiant Heart in the Temple District:
- Adventurer’s Mart. When you talk to Ribald, he’ll tell you about a mage named Khollynnus Paac, who can send the knights back to their home plane — for a mere 9000 gp. There isn’t any way to lower the price. If you agree to this deal, then Paac will appear in the sphere 24 hours later, and she’ll keep her end of the bargain. Before leaving, the knights will give you a
Golden Girdle of Urnst. You’ll also receive 45,000 xp.
- High Hall of the Radiant Heart. Inside this building, you’ll need to talk to Prelate Wessalen, who can be found in the back office. When he hears about the knights, he’ll offer to make them a part of the Most Noble Order of the Radiant Heart. When you relay this invitation to the Solamnic knights, they’ll reluctantly agree to stay in this plane, and you’ll earn 45,000 xp.
Quest 2: Apprentices
A day after your first meeting with Teos, he’ll return to the sphere with three apprentices in tow. He’ll then explain that your first task for the Cowled Wizards is to train the apprentices, whom he’ll introduce as Larz, Morul, and Nara. Luckily, “training” them in this case simply means ordering them to create items for you. These items can be dangerous for the apprentices, so you’ll have to weigh getting nice items with getting the apprentices killed. If you can keep all of the apprentices alive, then you’ll earn an xp reward at the end of the quest.
Each item will take four days to produce, and there’s a chance that the apprentices will fail in the task, even if they all survive. In general, the cheaper the item is, the more likely the apprentices will survive, and the more likely they’ll be able to craft it. If you want to find out the result as quickly as possible, then you can just rest in the sphere 12 times. The game will decide on the result when you assign the task, so if you want to guarantee a good outcome, then you’ll have to save before talking to the apprentices.
For the first item, the apprentices will need to create a minor magical item:
For the second item, the apprentices will need to create a spell scroll:
For the third item, the apprentices will need to create a major magical item (or not):
Robe of the Apprenti. This will cost you 250 gp, and one of the apprentices will die.
Ring of Wizardry: Reaching Ring. This will cost you 3000 gp, and two of the apprentices will die.
Staff of Power. This will cost you 10,000 gp, and all three of the apprentices will die.- Nothing. This will keep all of the apprentices alive.
One day after completing the final item (or not), Teos will return. If at least one of the apprentices survived, then he’ll hold a graduation ceremony in their honor. If all of the apprentices survived, then you’ll also earn 50,000 xp. If none of the apprentices survived, then Teos will comment on your “excellent tutoring” but nothing bad will happen.
Note: Keeping all of the apprentices alive will also give you a later benefit (see below).
Quest 3: Lord Ketlaar Argrim
A couple of days after the graduation ceremony, Sergeant Natula will come up to you and tell you that Teos wants to see you in the Planar Sphere. However, when you arrive, Teos will think that you summoned him there. Right after that, Sergeant Natula and a few of his friends will portal into the sphere and attack you “for the glory of Argrim.”
You’ll start out surrounded, and Teos will 
Dimension Door away, but the battle should not be too bad, provided you can keep the enemies off your mages. If you have trouble with the fight, then just buff up before entering the sphere. And be careful with area-affect spells. If you accidentally hit Teos before he leaves, then he’ll turn hostile, and you’ll botch the quest. You’ll earn 36,000 xp for defeating Sergeant Natula and his crew, but you’ll only find basic, non-magical gear on their corpses.
After the battle, Teos will return, and he’ll explain that Lord Ketlaar Argrim is “an anti-magic fanatic” who wants all mages dead, including you. But Teos will have a plan. He’ll tell you that Argrim can be found upstairs in the Crooked Crane at the City Gates, and that if you were to make him disappear, then his support would evaporate. Teos will then give you a Rune of Imprisonment.
Note: If you refuse to follow Teos’ plan, then he’ll kick you out of the sphere, and you’ll fail the quest.
When you confront Argrim in his room at the inn, he’ll attack you. He’ll only have two men-at-arms and two archers with him — that is, no magical support at all (as you’d expect) — so fighting him should not prove especially difficult. You’ll just have to be careful that you imprison Argrim rather than kill him, which means you might want to turn off your party AI for the fight.
Note: Any character can use the Rune of Imprisonment, so let your weakest companion use it on Argrim.
When you return to Teos in the Planar Sphere, you’ll earn 7500 xp for your work, even if you killed Argrim. Teos will also give you an update about 
Imoen, and he’ll tell you the single word “Spellhold.” You’ll visit Spellhold in Chapter 4.
Finally, if you successfully imprisoned Argrim, and if you kept all of your apprentices alive earlier, then after speaking with Teos, Morul will return, and he’ll ask if he can continue his studies in the sphere. If you accept him, then once a week he’ll produce some potions for you, possibly including 
Potions of Extra Healing, 
Potions of Invulnerability, and 
Potions of Master Thievery.
