As @The_Red_Queen says, using the Illithid tadpole dislog options to control conversations with NPCs will eventually result in you crossing a threshold, after which, the dialog options to use the tadpole are labelled [TRUE_SOUL}.
This does not affect the story in EA, but it seems likely to be the main plot device from Moonrise Towers onwards. Within EA, you can safely use the tadpole until you receive a class-based tadpole power for yourself and companions ( some of which are quite cool ), without actually becoming a True Soul or suffering any lasting ill effect.
When the game releases in August ( or whenever ), this may not be true; any use of the tadpole dialog options may have a lasting impact.
