From Bulbapedia:
“The only way to access the Sinjoh Ruins is by bringing an Arceus from an event distribution or the Hall of Origin to the Ruins of Alph. The event is triggered by having Arceus as the player’s walking Pokémon and entering the Ruins of Alph. (However, due to the Azure Flute never being officially released in Generation IV, it is not possible to officially obtain an Arceus from the Hall of Origin.)“
Arceus was obtained by loading up the Pokemon Toys R Us Arceus Distribution NDS Rom which distributed Arceus at participating Toys “R“ Us stores in the United States and Puerto Rico from November 7 to 15, 2009.
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