So far my sensation is that the tadpole doesnt transform people at all, but does something else to them. I havent found any sort of "illithid meter" yet, but i know that leaning on the tadpole's powers is supposed to, at least theoretically, allow it to influence you more in some way. Right now my assumption is that means increasing the DCs of checks to resist when the tadpole tries to control you. I ran into somebody else who had a tadpole, but they were dying. When they died, the tadpole came out and i tried to crush it, but i failed the check to assert my will and had to let it go.