I think it's actually just Tav. And by that I mean, ONLY Tav, and not PC Gale, for instance.
Everyone has their own special story thing going on, and Tav's revolves around the tadpole. Astarion about the people he bites, Gale's is about the books he eats, and Durge's about the Urges.
I really don't think that's what
this says: "The deeper you go through the illithid skill tree, the more powers you'll discover. But nothing comes without cost. While your companions can also consume parasites to gain their powers, not everyone in your party will agree to it, and your companions' perception of you can undergo a significant shift based on how you approach this opportunity."