The subjugated elder brain is an interesting concept.
What if the illithids who captured the player character are trying to save the elder brain in question by making the player being their pawn to join the absolute and discover what's going on and where it is located? It would explain why the tadpole is dormant. Cause illithids would be destroyed on sight by the absolute followers while apparent 'true souls' would be welcomed in open arms and could infiltrate the cult.
The cultists tadpoles are dormant too, otherwise they would all have been consumed and turned into mindflayers over the next 7 days after getting them.
