How to Beat the Netherbrain in BG3: Complete Final Boss Guide

Pre-Battle Preparation

Before entering the Morphic Pool (point of no return):

• Cast Heroes’ Feast (level 6 Cleric spell) — immunity to poison, fear, +2d10 max HP
• Cast Longstrider on everyone — +3m movement
• Cast Death Ward on everyone — prevents death once
• Drink Elixirs of Universal Resistance
• Every character carries Potions of Speed
• Summon all available allies before starting (elementals, planar allies, undead)
• The “Gather Your Allies” quest rewards you with Call Forth Allies actions — they’re powerful summons for this fight

Phase 1: Climb to the Crown

The arena is a vertical climb. Platforms rise and fall. Mind Flayers and Intellect Devourers guard each level.

• Don’t fight everything — your goal is the Crown of Karsus at the top
• Use Fly, Misty Step, and Dimension Door to bypass enemies
• Kill Mind Flayer Arcanists (they cast Magic Missile and Mind Blast — both stun)
• A mind flayer ally channels the Netherstones at the Crown (this takes several turns)

Phase 2: Inside the Netherbrain

After the portal opens, enter the Netherbrain’s psyche. Inside: 350 HP, 20 AC.

• The brain is vulnerable to Force damage. Disintegrate, Eldritch Blast, Magic Missile.
• 5 turn time limit before the brain unleashes its ultimate (instant game over)
• Burst everything. Save nothing. This is the last fight.
• Use Divine Smite, Action Surge, Level 6 spell slots, Scrolls of Disintegrate

Final Turn

At ~50 HP, the brain becomes resistant to everything. Use the Netherstones to weaken it, then finish it with your strongest single attack.