You should have found a reveal scroll in Evelyn's house, along with her diary.

Have you found the watcher statues?
Head south west from there[spoiler]to the beach
and keep going south
past a waterfall
until your characters mention that this place looks like the one described in the diary
and you need to use the reveal scroll
[/spoiler] If not, you need to go through a tunnel to get there...
The one where you fight a large robot.