The legendary Pokemon Arceus and Darkrai are now available to acquire in Pokemon Brilliant Diamond and Shining Pearl. However, there are different ways to get each one. One involves downloading the latest patch and owning Pokemon Legends: Arceus. The other is a more traditional Mystery Gift item available to download between new moons.
If you want to get Arceus in Pokemon Brilliant Diamond and Shining Pearl, you need Pokemon Legends: Arceus. You also have to have beaten that game and the one you’re playing. So first, you need to download the 1.3.0 patch for Pokemon Brilliant Diamond and Shining Pearl. Then, you need to make sure you own and beat the campaign in Pokemon Legends: Arceus. After that, when you open Pokemon Brilliant Diamond or Shining Pearl again, you need to get a National Dex and defeat the Elite Four. If you do, you can head to your room for an Azure Flute to use at Mount Coronet’s Spear Pillar.
Compared to that, Darkrai is much simpler to acquire in Pokemon Brilliant Diamond and Shining Pearl. If the time of year is between new moons, you open up Mystery Gift. This nets you a Member Card key item. Then, if you beat the Elite Four and have a National Dex, you can go to Newmoon Island, you can encounter and try to catch Darkrai.
Pokemon Brilliant Diamond and Shining Pearl are available for the Nintendo Switch, and the March 15, 2022 update lets people get Arceus in-game. Shaymin is also available in-game.