It's got to be in Gorgons Layer! People are mostly believe that it's at Atheon and the portals blah blah but if you think about it, the gorgon layer is in between Templar and atheon. Templar is past and atheon is future and gorgon is present. You don't get a reward during gorgon so it HAS to be there. When you get spotted by a Gorgon they technically erase you from time. So if gorgon layer isn't supposed to even BE IN TIME like the quote says " In between future and past but not in the present" or something like that. That's my theory.