The Legendary Pokémon Arceus is now available to encounter, meet, and catch in the Pokémon Diamond/Pearl remakes, Pokémon Brilliant Diamond/Shining Pearl. Revealed as part of the latest patch notes for the games, it has been noted that you can come across the ancient pocket monster, albeit with the catch of needing to have a Pokémon Legends Arceus save file to do so.
In order to begin the Arceus encounter, players will need to grab the Azure Flute, which will be found in their starting bedroom, and then take this item to the Spear Pillar at the peak of Mount Coronet, wherein the encounter will be prompted.
If you haven't played either Pokémon Brilliant Diamond/Shining Pearl or Pokémon Legends Arceus, you can check out what we thought about both games in our linked reviews.