Pichu and Arceus In-Game Events

The Official Pokemon Website and leaked Famitsu scans revealed new information about the role of Pichu and Arceus in Pokémon Heart Gold and Soul Silver. The Pikachu-Colored Pichu that will be distributed via Mystery Gift this summer for the 12th Pokémon movie will allow you to interact with a Jagged-Eared Pichu in Pokémon Heart Gold and Soul Silver. However, it is noted that the Pikachu-Colored Pichu must not be evolved in order to activate this event. You can evolve it, but if you do, you will not be able meet the Jagged-Eared Pichu. There is no word as to whether or not a new Pichu forme will be introduced.
In addition, more info regarding an in-game event activated by the 12th movie Arceus has also been announced for Pokemon Platinum. If you have this Arceus in your party, you can find the Hiker in Oreburgh Mine and he will tell you about his studies regarding philosophy and legends. As he leaves, he will give you the Flame Plate. You can then meet him at the Canalave Library where he will tell you the legend of the creation of Sinnoh. Furthermore, the book that he placed in the library will be updated with the text from all the plates.
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