Shiny Rock Outside Victory Road?

[MrWh00pee]

Ok, once I obtained my national dex I went the alternate way in victory road the one that was closed off the first time you went in by a guy. I went through it and led the girl to the end and she said she wanted to see what pokemon came to the shiny stone or w.e so I then searched for it and found the shiny stone but no pokemon? Can someone tell me what this rock is for it says when I touch the rock "The Surface Is Unmarked. It is polished like a mirror"

[wakachamo]

It's the door to getting Shaymin. However, you need to get an item called the "Oak's Letter" and then go to that rock. To get such an item, you need to go to a Nintendo event. However, there's no such event so far.