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The only to get there is to activate Darkrai event.. There's a house at canalave which you can't get in, unless you activate it. You will have to sleep on the bed and you will be transported to New Moon Island.. There you will encounter a wild Darkrai.
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You need the Adamant and Lustrous Orbs in Mt. Coronet. After getting 'em, talk to the guy in the large house in Celestic Town. Head back to Spear Pillar and you will see a blue portal (Dialga). After capturing, exit the area and head back in and you will see a pink portal (Palkia).