Upper World Achievement
Enter the world of gods and ancestors
Story-related; cannot be missed.
At the end of the middle world you'll come to 7 large wooden posts with eyes. They will light up and show you the order they must be activated in. Use your staff and mask to activate them in the correct order and then a bird will land. Hop on its back and this achievement will unlock.
Refer to the "Sunset" achievement solution for a video walkthrough of the whole game.