Make your way northeast through the Dragon's insides. There's not much to do at first, though when you reach an Origin Mirror you will need to look around for an Exorcising Arrow. To find the key you have to drop off of the main path and into the pool of water that runs along the bottom of the dragon's belly (just south of where the Origin Mirror is). When you have the key, use Waterspout to make a fountain that'll let you return to the upper walkways and continue northeast. Just ahead, double jump up a ledge to find a lock that the Exorcising Arrow will destroy. Just beyond the locked passage is a gooey mess that's surrounding the Dragon Orb. Unfortunately, you can't get just grab it.