You must catch every Pokemon, that's 493!! Good luck!
Oh yah, and it is imposible to catch, Deoxys, Jirachi, Arceus, Shamin, Daraki, Celebi and Mew with out going to a Nintendo event or hacking!
lier.you can use the Pokemon modifier cheat.if u want it look up "what is the best cheat for Pokemon diamond.i have it there.
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To catch every Pokemon, you need action replay for the few last diamond legendaries. You will need Pokemon fire red/leaf green and emerald. Migrate them to Pokemon diamond. In Pokemon diamond, fill up your pokedex by catching Pokemon normally or with poke radar, or use gts. It will take a long time to catch all Pokemon
you would have to set loose some Pokemon in order to make room for other Pokemon.
You become an uber nerd;)
Do you have Giratina? Do you have a national pokedex? did you get Heatrans? If you have a national pokedex you can go to Celestic Town and go in the cave and observe the inscription. Cynthia says that it may not be the three lake Pokemon. maybe it's Dialga, Palkia, and Giratina. I dunno. I'm kinda at a loss, too... Just try to fill up your pokedex, i guess...
You can look up the pokedex for the hoenn region