You need to get an Azure Flute. Use it at Spear Pillar. There are a couple ways to get it.
1. Action Replay
2. Mystery Gift
3. Event
4. Use dive might be dig at where you would find Giratina. (Heard it from a friend.)
Once you get one you cant get another and it only works 1 time.
hall of origin
TO GET ARCES HOPE THAT HELPED A LOT
pokemon diamond. get the azure flute play it at the top of mout cornet. or spear pillar. it is at lv 80
You can't
Trade from Diamond, Pearl, or Platinum
Good question the answer is Mew, Mewtwo Arces
well you can get Shamin Arces Darkrai Creslia and Regigagis
get the azure flute and then go to where you met dialga and play it
only with action replay!!! there is no way without action replay
ponyta and rapidash.
arces,palkia,dialga,garitina,ect.
I think it's Arces IDK if it is but I'm thinking it is but good luck trying to find the answer your looking for alright!!!! POKEMON!