You’ll need to own a copy of either remake as well as a copy of Pokémon Legends: Arceus. Furthermore, the games must be up to date with version 1.3.0, which was released on March 15.
Getting Arceus requires you to complete the game of the same name, or at least have the main missions done. In Brilliant Diamond or Shining Pearl you’ll need to have entered the Hall of Fame and obtained the National Pokédex.
When that’s all done, simply return to your room in Diamond or Pearl and you will get an item called the Azure Flute. After that, you need to go to the Spear Pillar at the top of Mount Coronet, whereby Arceus will show up, and you’ll have to battle the legendary Pokémon to capture it.
It’s a similar situation with Darkrai, only you’ll need to use the Mystery Gift feature in Diamond or Pearl, which The Pokémon Company explains is “available between the new moons”. You’ll still need the National Pokédex, but finding Darkrai means travelling to Newmoon Island. The company added that these legendary Pocket Monsters are “sure to make incredible additions to your team”.
Even though the latest game in the series was only just released, we’ll be getting another new adventure this year, with Pokémon Scarlet and Pokémon Violet coming later this year, exclusively for Nintendo Switch. It might seem a lot to have two new Pokémon games in one year, but Legends Arceus was actually originally intended to be released in late 2021, but was delayed to January 2022.