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.
I doubt this, the Flayer onboard the Nautiloid thinks you are his thrall, after all.
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Auntie Ethel says the parasite is enhanced by Netherese magic.
Oh, right! There is *that* too. Still, that only supports a how, not necessarily the who. It *does* tie into Gale's story though, the one I couldn't tie in earlier.