The Crown of Karsus
When you reach Wyrm's Crossing, you can run into your old friend Raphael at Sharess's Caress, an establishment north of South Span Checkpoint. He'll be in a room on the third floor labeled the Devil's Den. Speak with him to hear an offer he wants to make; he wants you to deliver the crown worn by the elder brain in exchange for a tool that will allow you to free Prince Orpheus from the Astral Prism. Regardless of what you choose here, Raphael gives you plenty of information about the Crown of Karsus. Speaking to Gale about the crown prompts him to suggest looking for information at Sorcerous Sundries, a shop in Baldur's Gate.
When you reach the Lower City, the shop will be a short walk southwest of the Basilisk Gate Teleport Sigil. Additionally, it will be marked on your map, making it easier to find. Head inside and speak to Tolna Tome-Monger, found at the back of the shop.
She'll list off a handful of their more dangerous books, you need to ask for 'The Annals of Karsus: A Netherese Folly'. She refuses to give it to you, but if you succeed in a speech check, you can get her to tell you it's in a vault accessible through her office. She won't give you access to her office, but this is a good lead. For now, head upstairs to Tolna's office. You'll need to lockpick the door, but it's hard to avoid being spotted, so cast Darkness on the area around the door so you can discreetly enter the office.
First, grab the Sorcerous Sundries Basement Chest Key. Then, interact with the Clasped Book in the round bookcase to open a portal to the basement, then enter. Head south into a room with three other doors. Two of them are locked, but you can enter the Silverhand door. To open the Karsus door, head through the Abjuration door, then the Silver door. This room will also contain a lever, so activate it and use any other door to head back to the start.
Upon entering the vault, you'll find the Annals of Karsus waiting on a shelf. Reading it reveals that the crown worn by the elder brain is indeed the crown forged by Karsus. Combining it with the three Netherstones can restore its power, at which point Gale can use it or destroy it.
When you next visit your camp, Gale's friend Elminster will be visiting. He speaks with Gale about his failure to destroy the elder brain and the Crown of Karsus. As a result, Mystra has summoned Gale to her shrine in the Stormshore Tabernacle.
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