You'll need to migrate it to the Pal Park from Pokemon Sapphire, Ruby, or Emerald.
you can not you must trade or migrate
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Regice is not obtainable in Pokémon Diamond.Regice isn't in diamond.
you cant get it on diamond or pearl!
you have to trade from diamond or pearl
the only way you can get regice in heart gold is by trading from Pokemon platinum,diamond and pearl
you need regice, regirock, and registeel in your team when you fight him