Once you’ve obtained Noah’s Lute from the Temple of Time, resurface. Go along the coastline on the west side of the continent, then go right when possible into the mountains.
Go into the cave and use the Lute to wake Unei up . After a scene, you’ll learn what you need to be doing and Unei will join the party, as well as hand over the Fang of Fire .
Now, start flying in the overworld. Go to the center parts of the Saronian continent and, on its west coast, you’ll find the mouth of a river that splits into two, making a triangle delta, with trees between the forks. From there, follow the river to the east and land in front of the cave and head on in.