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).
Funny, that is definitely not where I found the star stone to unlock the teller of secrets. I'm pretty sure it was somewhere in Cyseal TBH.
Yeah it seems that the rooms at the end of time are unlocked in a certain order regardless of which order you find the star stones. from what I've read the 5th star stone you find unlocks the teller of secrets.
