well actually you go to turn back cave if you get in the chamber with the portal in 3 rooms it will be there then go in the portal and get to the end where you'll find a griseous orb ( platinum orb)...
The Platnium Orb has been translated into the Griseous Orb. After you have defeated the Elite Four and Champion, go to the Turnback Cave. You have to go through a puzzle. Once you complete it, you...