if you have Action replay then you have to have the azure Flute cheat and enable mystery gift cheat for it to work, which doesnt mess up your game,
with and event which i think is too late you have to got to a toy store and attend the event is they are hosting it
but the last way is to use action replay and to get it to work is to go to the spear pillar and it should ask you if you want to use the flute
hes lvl 80 so save before fighting him and good luck
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First, you have a friend give you a level 99 Pokemon in the union room, then make sure the Pokemon isn't holding anything, finally, just leave it in the daycare until it's level 100, then when you take it out, it will be holding an azure flute. It may take a while, and make sure that the Pokemon isn't related to any Pokemon that youre the OT of(father, daughter, neice, etc.)
Arceus - Supposedly it shaped the galaxy in the game. You can catch him by going to a Pokemon event or using and A.R. to get an Azure Flute. Go back to Spear Pillar where you go to the distortion world and the flute will make noise. Some stairs will appear taking you up to Arceus...
You need the azure flute. then you go to spear pillar.the Azure flute will begin to make noises. blow into the Azure flute and a glowing stairway will appear. walk up the stairway and you will find a diamond platform. that is the hall of origin. go to the back and you will find Arceus, the creator of all Pokemon! to catch Arceus, at the start of the battle, throw a master-ball and POOF!!! HES YOURS!!!
If you used it once, then it should work. If you hacked or cheat to get another Azure Flute and went back to Mt. Coronet to where Dialga and Palkia were, you cannot be able to do it again. Make sure it was your first try using the Azure Flute and you are where the Hall of Origin is.
To make the stairway appear you need a special key item called the azure flute. It can be obtained from a Nintendo wi-fi event.