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.
