My party is level 10, and I'm still not even sure how to unlock the teller of secrets. So far as I know, I've done everything through (save for returning the staff to the mayor, so maybe that's the clincher).
Here's where the star stone is:
in the immaculate trials dungeon, in the first door on your left as soon as you enter the dungeon. Initially it's locked but you can pick it if you don't feel like getting the rot and jumping through the hoops for the immaculates(I know I don't).
