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actually you catch them right after you beat the elite four in platinum. Go to celestic town and talk the elder of the town. She is in the big brown house and when you to talk to her she'll read you a book about them. Then go to the spear pillar(if you dont remember its the place where you entered the distortion world)then get the two orbs adamant orb,and lustrous orb. youll need those orbs to summon them. if you dont know where they are just go on youtube and search it up. Then when you go to the top of the spear pillar there will be a blue diamond. inside will be dialga once you battled it and hopefully you got it get out of there and go back to the spear pillar and this time there will be be a pink diamond and inside is palkia. Hopefully this helped