For the end of a mysterious valley.
At the end of a valley you row by boat through a small tunnel and reach the Skull Valley,
there you can find some ruins of a past settlement, which are already overgrown and lots of lava.
Finally the skull, which I finally got a halfway good one after several attempts.
Beneath the skull is a door that may lead to a temple. In my case, however, there is only an empty room.