How do you get the legendary Pokemon in Pokemon diamond?

Answer 1

this is how...uhmmm.... certain ledgandaries you get from gba games a.k.a: zapdos, arcticuno, and moltres and dialga and palkia are in the game!!! (and some use Nintendo events, action replays, and gamesharks!!)

hope i helped!! ^.^

Answer 2

In Pokémon Diamond in order to get Legendary Pokémon such as Dialga you will need to encounter it in Spear Pillar and battle it, you can do battle with Uxie in Lake Acuity, Azelf can be found in Lake Valor, Mesprit can be found in Lake Verity but once you interact with it you will see it fly off and then it will become a roaming Pokémon at which point you will have to track it down during its location entry that will be registered in your Poke Dex, Heatran can be found on Stark Mountain, Regigigas can be found in the Snowpoint Temple but will only awaken for battle if Regirock, Regice and Registeel are in your party, Giratina can be found in the Turnback Cave and you can encounter Cresselia on Fullmoon Island but once you interact with it you will see it fly off and then it will be a roaming Pokémon that you will have to search for.