Pokémon Diamond and Pearl/Route 224

''Route 224 is the Route that opens up to you after you defeat the Elite Four. it is essential in the finding of the Legendary Pokémon Shaymin, though it is currently unavailable through normal means until Nintendo holds an event.''

Pokémon
The Trainers here may give you some trouble with Pokémon at levels 50+. This is the toughest Route in the game, so prepare accordingly.