I would prefer it as another dialogue option instead. It seems to me that using the tadpole too much causes some currently unknown negative effects. Don't remember what exactly happened but there was something mentioned about losing something you will never get back.
Mechanically that means nothing and is storyline floof. A Great Old One warlock would embrace this, this is the source of their power. Yo uhave to consider the motivations of a warlock to begin with, they have a pact with some super powerful patron to recieve some of those powers. Having a tadpole already in your head would be a boon for a warlock who sought powers of similar patrons.
