After you get 7 gym badges, he will be at spear pillar.
Spear pillar on Mt. Cornet
Dialga is in spear pillar
on the top of mt. cornet with cyrus.
top of mt cornet
you don't need an ar code for dialga in diamond just follow the story and befor the 8th gym leader you go to mt cornet and get dialga at spear piller
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If you're playing Diamond, the Pokemon in Mt. Coronet should be Dialga. You have to get the 7th badge before you are able to get to the top of Mt. Coronet where Dialga is though.
Dialga is at the top of Mnt Coronet.(Go from Heartrome city side!)
You have to get the Legendary Pokemon for diamond or pearl. They are at the top of Mt. Mcorent. You have to get the Legendary Pokemon for diamond or pearl. They are at the top of Mt. Mcorent.
You get Dialga at Spear Pillar on top of Mt. Coronot.
The peak
you have to use ar or go to mt,cornet and kill dialga