Sinnoh’s Pokédex in Pokémon Brilliant Diamond and Shining Pearl is much more forgiving and achievable than Galar’s in Sword and Shield. As in the original game, there are only 150 Pokémon in the Sinnoh Pokédex. Furthermore, you don’t have to “catch ’em all” – you merely have to see them all, through battles with trainers or in the wild. For those who have completed their Sinnoh Pokédex and want more of a challenge, Brilliant Diamond and Shining Pearl offer an additional hurdle: the National Pokédex.
The National Pokédex is part of Brilliant Diamond and Shining Pearl’s post-game. To get the it, you first have to beat the Elite Four and become Champion. You also have to have a complete Sinnoh Pokédex. With these two achievements under your belt, head back to where it all began, at professor Rowan’s Lab in Sandgem Town. Speak to Professor Rowan. Upon learning you completed your Pokédex, Roman will prompt an appearance from his pal, Professor Oak. Professor Oak will then upgrade your Sinnoh Pokédex to a National Pokédex.
The National Pokédex will add 300 new Pokémon from Generations I, II, and III into the wilds of Sinnoh. It will also unlock Ramanas Park, where you can catch legendary Pokémon from these older generations.