Story-wise, the Illithid is described as continuously struggling to keep the Netherbrain restrained. If the Illithid dies before the encounter ends, the Netherbrain should immediately break free and enslave everyone.
Instead, the combat mechanics continue to process Synaptic Consumption independently. The Netherbrain can still die from its Dexterity reaching 0 several rounds later, despite the character responsible for maintaining the Chains of Karsus already being dead.
I look forward to seeing the following plot:
Illithid teammate dies
↓
The Netherbrain breaks free from the Chains of Karsus. The Netherbrain enslaves tadpoles and turns them to mindflayers.
↓
Netherbrain proclaim a victory speech: "Thralls! I am free! blablabla (DEX → 0)
↓
Netherbrain: "what?" Dies immediately.
