What is behind the fat guy in victory road?

Well,I've heard rumors.....One says its a new route with other pokemon ,another says its arceus ,another says Its the area with shaymin (once you have oaks letter),and the last one says its all those things,but you cant go to the actual part for the legends without the Items. All I know is surely true is the there is some cool stuff behind him..and that you cant go past the shiny stone,which acording to legend,is where arceus rests and shaymin sleeps.............