New story details are in the article from the latest Dragon+ magazine:
https://dnd.dragonmag.com/2020/03/02/in-the-works-baldurs-gate-iii/content.html“The more you use your tadpole by selecting it from the options within dialogue, the stronger the influence of your tadpole becomes. But the more you use it, the quicker you unlock your powers,” says Larian Senior Designer Edouard Imbert.
“It’s that duality we’re introducing. It’s comparable to Bhaalspawn, which is a nice link to Baldur’s Gate and Baldur’s Gate II. We want players to be wary this thing is still inside them, and the fact that they can turn into a mind flayer is still there,” warns Smith.