National Pokédex

The National Pokédex has been brought back in Pokémon Brilliant Diamond & Shining Pearl. This variant of the Pokédex tracks all the Pokémon available in the game from Bulbasaur up to Arceus and is given to you by Professor Oak after you have shown Professor Rowan a completed (Seen) Sinnoh Pokédex, and when you get it, more of the game unlocks.

If you battle all the trainers in the game you should be able to see the majority of the Pokémon in the Sinnoh Pokédex, though you can see the other version's Legendary at Celestic Town

When you have the National Pokédex, far more areas will unlock. You'll be able to go to the other parts of the Battle Zone, where many wild Pokémon not natively from Sinnoh will roam. In addition to that, you will gain access to the PokéRadar which lets you find more Pokémon, and Swarms will start to appear. More Pokémon will also appear in the Grand Underground.

Other trainers will start to include more Pokémon in their teams for their rematches including Gym Leaders and the Elite Four. Finally, you'll be able to go to Ramanas Park in order to find a variety of Legendary Pokémon

After you have completed the National Pokédex (excluding Mythial Pokémon), if you go the Grand Lake Hotel and go to the leftmost building, you'll find the two game directors. If you speak to Ueda on the left, you will receive a Shiny Charm and if you speak to Masuda on the right, you will get a special diploma.