Larian Studios Forums Baldur's Gate III Baldur's Gate III - General Reducing the Netherbrain's Dexterity to 0 instantly kills it and skips the surrender sequence

That's a core rule going back to D&D 1e. Unless otherwise described in the effect that causes it, reducing an attribute to zero kills the character. How that physically manifests is either described in the spell or monster entry or up to the DM. So, a collapse of the nervous system is a pretty good guess in this case.