It's Shar who tampered with the tadpole using shadow magic and this magic is what shields your character from the absolute's powers. Which is why the goblin priestess, once she sees "the shadows" in your mind when you talk to her, tries to kill you.
I think the explanation is simply that she notices something inside the person she is talking with. She is not aware of having a tadpole so she does not know what it could be and calls it a shadow. I think there was something mentioned about netherese shadow magic. That would point to Shadovar and Shar but she seems to be opposed to the absolute. As for Shar
Dialogue for Act 2 reveals that Astarion is a follower of Shar. I don't see a connection between Lae'zel and Shar.
I sometimes use thought experiments. I don't necessarily believe in every idea I post for discussion on this forum
