Okay, I played through Y version as blindly as possible. I knew the starters and most of the stuff released in CoroCoro. I stayed away from /vp and serebii starting the day the game leaked in Canada and Italy.

So, I saw the description of terminus cave on the map of there being a "monster" and, excited, I went straight there after finishing the league and found the glorious Zygarde. I had no idea what I would find and the feeling was amazing.

But then things went a little sour. I was sure something would be at Sea Spirit's Den so I went there next and it was empty. "That's fine" I thought. I'll probably be able to activate it soon. I then went to reflection cave because there's a really bright, glowy hidden area that I was 100% sure would house a legendary Pokemon. It was empty. Again, I thought it would activate soon.

I looked some more, found no more legends anywhere and no sign of a method for activating them. So I checked serebii for the first time. When I found out Sea Spirits Den is just for the kanto birds and there are no other legends after Zygarde.

Feeling a little disappointed at that. Love the game still though.

But that begs the questions: what is the area in reflection cave for?