Claiming the Sphere is the Mage Stronghold Quest in Baldur's Gate II: Shadows of Amn. It is the questline given to Gorion's Ward if they are a Mage, Specialist Mage, Sorcerer or Wild Mage and complete the Lavok's Sphere dungeon.
After returning to Athkatla, a Cowled Wizard emissary named Teos will appear and offer Gorion's Ward a bargain: if they are willing to prove they can be "responsible" (read: obedient to the Cowled Wizards), then they are willing to let them maintain control over the Sphere and all its many secrets. The questline, then, is a series of mini-quests about proving yourself worthy to keep the Sphere.
Task One: Returning the Knights of SolamniaEdit
The first matter is simple; find a way to get the Knights of Solamnia out of the Sphere. If asked for advice, Teos comments that they could find an agreeable home with the Order of the Radiant Heart, which is very much a local equivalent of their Order. Alternatively, Ribald Barterman of the Adventurer's Mart keeps unusual company, and quite powerful ones too; he may know a wizard strong enough to planeshift the Knights back to Krynn. The third option, which Teos personally favours, is killing the Knights.
Speaking to Prelate Wessalen in the Athkatla Temple confirms that they would be happy to accept the Knights; if the player tells them this, they will move out to the Order's temple, somewhat sad, but willing to forge a new life for themselves here. The player is rewarded with 45,000 XP for choosing this option.
Ribald Barterman, meanwhile, does know somebody who can help - but she wants 9000 gold pieces in exchange for it. If the player pays, then she will teleport to the Sphere one day later and send the joyous Knights back to their own world. Your reward for this is 45,000 XP and the Golden Girdle.
Task 2: ApprenticeshipsEdit
Once the Knights have left, Teos returns with your first true assignment; training three young wizardlings to become full-fledged Cowled Wizards. Morul, Larz and Nara must undertake three lessons in the Planar Sphere, bending their talents towards proving themselves in three areas, with end-goals set by the player. However, because of the deadliness of magic, it's possible that they may not survive the teaching.
Lesson One: CraftingEdit
Creating the Dagger of CHARNAME ensures that all apprentices survive for the next lesson.
The Wand of the Apprenti has a strong chance of either failing outright or killing one of the apprentices if they are told to create this item.
Attempting to create a Ring of Protection +1 will certainly kill Larz in the process.
Lesson Two: ScribingEdit
In general, the higher the spell's level, the more likely that one or more apprentices will be killed scribing it. Mislead is the safest option.
Lesson Three: EnchantingEdit
For their final lesson, the three apprentices must attempt to create a more powerful magical item, offering the choice between the Robe of the Apprenti, the Ring of Wizardry, and a Staff of Power. The player may declare that they are not ready to tackle such potent magecraft yet, but otherwise their training is complete, instead of choosing an item.
The Robe of the Apprenti will kill Larz if this option is selected.
The Ring of Wizardry will consume Larz and Nara's lives if they attempt it.
The Staff of Power is beyond the power of the three and they will all die in the attempt. There is a chance, however, that they will create the item in the process; death is certain, that the Staff will not be created is not so certain.
If the player declares they are not to attempt this lesson, then they all live.
Once the three lessons are complete, Teos holds a short graduation ceremony. He's amazed if all of them survived, nonchalant about losing one or two, and chides you if you killed them all. If all apprentices survived, you receive 50000 XP. Otherwise, you get nothing.
Task 3: AssassinationEdit
Two days after the graduation ceremony, Sergeant Natula will approach you in Athkatla and tell you that Teos wishes to speak to you in the Planar Sphere. When you arrive, however, Teos is confused; he was told that Gorion's Ward wanted to speak to him. Both parties are then ambushed by Sergeant Natula and some of his men.
Once the ambush is slaughtered, Teos explains that Natula was a pawn of an anti-magic fanatic, one Lord Ketlaar Argrim , who wants to see all mages driven from Athkatla, including the Cowled Wizards. He gives Gorion's Ward a Rune of Imprisonment and charges them with using it to make sure that Lord Argrim literally disappears.
Lord Argrim and his defenders are hiding in the Crooked Crane Inn. Once his men are dead and he is banished, Gorion's Ward may return to the Planar Sphere. There, Teos will tell them that Imoen was taken to Spellhold and that they have passed all of the Cowled Wizards' tests; the Sphere belongs to Gorion's Ward, now, and Teos leaves.
The quest is now complete. If all of your apprentices survived, Morul will teleport back to the Sphere once Teos leaves, asking if he may remain as your apprentice. Accepting will give you access to his weekly potion batches.