How To Unlock Arceus And Darkrai In Pokemon Brilliant Diamond And Shining Pearl
If you think your game of "Pokemon Brilliant Diamond" or "Pokemon Shining Pearl" is lacking in legendary Pokemon, the new 1.3.0 update gives you a chance to change that. In addition to the nightmarish fury of Darkrai, you now can add the divine power of Arceus to your team roster — so long as you meet the proper criteria. This pair of Pokemon are classified as "legendary," after all. They can't just pal-around with every Pokemon Trainer who asks nicely.
To begin, you may need to do a bit of prep work, depending on how far along you are in your game of "Brilliant Diamond" or "Shining Pearl." In particular, you'll need to have beaten your game's main story. This does not mean you'll need to complete every single task in the game. It just means you need to reach the end of the main plot. You'll also need to keep an eye on the calendar if you're going after Darkrai, since its gimmick is tied to the actual phases of our planet's Moon, outside of the game, here in reality.
Figuring out Darkrai
Darkrai can be found in your dreams. No, seriously, you have to fall asleep in-game in order to do it. But first, you need to unlock the National Pokedex, which means seeing (though not necessarily capturing) all 150 Pokemon in the game, and get into the Hall of Fame by beating the Elite Four and becoming the new Pokemon Champion. Then you have to get ahold of the Member Card, which is available via the Mystery Gift menu option "between the new moons." Meaning that, here in the year 2022, you've got a window of opportunity between April 1 and April 30. Plenty of time to get into the Hall of Fame and unlock the National Pokedex at least, right?
Once you've got the card you have to return to Canalave City and visit the house with the blue roof to the north of the Pokemon Center — the one you might remember being locked the last time you visited, earlier in the game. It'll be unlocked now, so you can go in and speak to the NPC waiting inside. Follow the prompts and go to bed, and you'll find yourself on Newmoon Island. From there you can wander around until you find Darkrai, but be prepared for a tough fight against a legendary Dark-type Pokemon (who's weak to Fairy, Bug, and Fighting attacks).
Finding and fighting Arceus
Finding Arceus also requires that you, the Pokemon Trainer, be part of the Hall of Fame by beating the Elite Four and unlocking the National Pokedex (see all 150 Pokemon). Unlike Darkrai, you'll also need register save data from a completely separate video game on your Nintendo Switch. Your Switch needs to have the game "Pokemon Legends: Arceus" loaded to a point where you have game save data in your device's memory. You don't have to complete "Pokemon Legends: Arceus" for this to work, but you do need to have played and saved your game in some capacity.
So long as you meet all those prerequisites (and have "Pokemon Brilliant Diamond" or "Pokemon Shining Pearl" updated to version 1.3.0), you'll be able to go back to your room from the very start of "Brilliant Diamond" or "Shining Pearl" and find the Azure Flute item. Once you have the flute, visit the Spear Pillar at the top of Mount Coronet and Arceus will be waiting.
Like Darkrai, you should be prepared for a tough fight. Particularly since Arceus is basically (and canonically) a Pokemon god that participated in the creation of the entire world. It is important to note, though, that Arceus is weak to Fighting moves, which seems odd for a divine being.