Location of the Clairvoyance mutation.
To get in, you must drop in from Luskentyre Plateau. Your path will be blocked by Zebediah. You can either farm 200 church rep to gain entrance, or get creative. Bring a rope, as behind him there is a climb rated as nearly impossible.
There is also an opening to the Cave from the lower canyon -- however, it's too high to reach from below.
Inside the cave you will find Zhobelia, a ghost who blocks your path to true vision. Kill him with EMP damage.