The only way to get a Deoxys in Pokémon Diamond is to attend a special event or use the Pal Park. The Pal Park allows players to import a Deoxys from Emerald, FireRed or LeafGreen games. Simply visit the Pal Park located on Route 221 in Sinnoh or in Fuchsia City in Kanto.Continue Reading
The Pal Park is only accessible in Pokémon Diamond after players have obtained HM03 and the Relic Badge. Players will also need to see all of the Pokémon in the Sinnoh Pokédex and receive the National Pokédex. Once these requirements are met, Professor Oak will meet the player at the Route 221 entrance and walk the player through the trading process.
Once in the Pal Park, players have the option to “migrate” Pokémon from a previous game version. Only one Pokémon per day can be transferred from a previous generation. Select the Deoxys, and it will be sent to the Pal Park.
Once transferred, the player will need to find his Deoxys in the mountain section of the Pal Park. The player can then catch it using a special ball.