The one in the snow area across the water? For that you need the owl forms glide ability upgrade. You can buy it in the dojo to reveal hidden gliding spots.
thanks for the help i got the key but is the only thing in the dragon sanctuary the bad ending? when i touch the blue orb whit the dragon tf it says that "nothing happens" i tried kill the demon for the dragon tf but sill says that "nothing happens" sorry for asking for so much help
