A character must have the Illithid passive Ability Drain.
At the beginning of the encounter (5 turns remaining), cast Synaptic Consumption on the Netherbrain (DEX 10 → 5). Synaptic Consumption then reduces the Netherbrain's Dexterity by 1 at the end of each round, just before the Netherbrain takes its turn. However, Synaptic Consumption alone is not enough. On the final turn, the Chains of Karsus break before the fifth Dexterity reduction can occur, leaving the Netherbrain at DEX 1 instead of 0.
To overcome this, hit the Netherbrain once with a bow (or any DEX-based attack) during the fight while having the Ability Drain passive. This applies Ability Drain Dexterity, reducing DEX by 1.
During my testing, I also confirmed several important mechanics:
1. Synaptic Consumption and Absorb Intellect share the same mutually exclusive status and cannot both be applied to the same target.
2. Ability Drain can coexist with Synaptic Consumption.
3. Different Ability Drain effects (e.g. Strength and Dexterity) can coexist on the same target.
4. The Netherbrain is not immune to ability scores being reduced to 0.
5. Death caused by reducing an ability score to 0 bypasses the Netherbrain's normal death sequence. There is no surrender sequence and no final cutscene. The game skips directly to Withers' epilogue reunion party.
