edit: the Hag mentions removing a tadpole before, but I didn't get the impression it was a true soul. Our tadpole is drenched in shadow magic, I'm assuming that all of them are.
Not all of them. We can confirm inside the goblin fortress, while confiding to the goblin priestess as she communes with the absolute about our tadpoles and reading our minds, that there's a tadpole inside every "True Soul's" brain, but somehow none of them seems to be aware of the fact.
