How to Solve the BG3 Brain Puzzle (Necromancy of Thay)
The bg3 mind puzzle everyone searches for is the Necromancy of Thay sequence: a hidden cellar, a riddling mirror, a cursed gem and a book that fights back when you read it. This guide walks the whole chain in eight steps, from the Blighted Village apothecary down to the Act 3 payoff. Tap each step to reveal exactly what to click, what to answer and which saving throws to prepare for.
Step 1: Find the Cellar
Where does the bg3 brain puzzle start?
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In the Blighted Village in Act 1. Enter the apothecary's house just south of the village waypoint, read the Apothecary's Ledger on the counter to start Search the Cellar, then open the wooden hatch behind the counter.
Step 3: Answer the Ornate Mirror
An enchanted mirror blocks the way and starts asking questions. What do you say?
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The mirror belonged to the necromancer Ilyn Toth, so answer like his ally.
- When it asks about Szass Tam, condemn him: call him a foul lich who deserves a thousand more deaths.
- When it asks what balsam ointment is for, say you read the doctor's journal and that he used balsam to clean wounds.
- When it asks what you would see in the mirror, give an honest personal answer; most sincere responses pass. The mirror then swings open.
Step 4: Reach the Book
What is waiting past the mirror?
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Ilyn Toth's hidden laboratory. The Necromancy of Thay sits on the eastern side, guarded by pressure plates and gargoyle traps. Disarm or step around the plates, then grab the tome.
Step 5: Get the Dark Amethyst
The book is sealed shut. What opens it?
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The Dark Amethyst, a cursed gem that fits the slot on the book's cover. Find it in the Whispering Depths, reached through the well or the blacksmith's basement in the same village. The gem lies in the lair of the Phase Spider Matriarch, so fight her or sneak the grab.
Step 6: Open the Book
How do you actually open the Necromancy of Thay?
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With the Dark Amethyst in your inventory, examine the book and choose to place the amethyst in the slot on its cover, then select Open the book. Pick your most Wisdom-proof character before you do; the reading is where the real bg3 mind puzzle begins.
Step 7: Survive the Reading
What happens while reading, and how do you survive it?
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Turning each of the three pages forces a Wisdom saving throw at DC 10, then 15, then 20.
- Failing once inflicts Whispers of Madness: disadvantage on Wisdom checks until a long rest.
- Failing twice inflicts Baleful Knowledge, a curse that only Remove Curse clears.
- Buff before reading: Warding Bond, Bless, or a high-Wisdom reader like a cleric makes the saves manageable.
- Turn all three pages and the reader gains the Forbidden Knowledge passive, widely reported as a permanent +1 to Wisdom saving throws. The book then refuses to open again for now.
Step 8: Finish It in Act 3
The book slammed shut after three pages. Is that the end?
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No. In Act 3, pull the Tharchiate Codex from the vault under Sorcerous Sundries and read it. The Necromancy of Thay will open again for your Forbidden Knowledge reader; pass one final DC 20 Wisdom save and you learn Danse Macabre, which summons a pack of ghouls to fight for you.
Frequently asked questions
Should you read or destroy the Necromancy of Thay?
Reading it is the only way to get Forbidden Knowledge and, later, Danse Macabre. Destroying it requires radiant damage and spawns three hostile Shadows, with no comparable reward. Most players read it.
Can Astarion or another companion read the book instead?
Yes. Any companion can be the reader, and the buffs belong to whoever turns the pages. Pick the party member with the best Wisdom saving throws.
Is the bg3 brain puzzle related to the illithid tadpole?
No. Despite the nickname, the brain (mind) puzzle is the Necromancy of Thay sequence: the cellar, the ornate mirror and the saving-throw reading. Your tadpole plays no part in it.