Pokemon Diamond

How to get all the Pokemon:
Sorry, you cannot get all in Pokemon Diamond.
You have to trade Pokemons from your GBA Pokemon games.

You can catch all of the 4th generation monsters, but just
about 50 original Hoenn, Kanto, Johto pokemon.

In this game there is a world called the dark world it can only be
accessed by beating the elite 4 then going back to the first door and in
front of that door use surf and you will be transported to the dark world,
you can catch darkrai here and many others. To get out you must have
a pokemon that can use dig or fly or..... you will be trapped there....