The nightmare Pokémon Darkrai will only appear in Pokémon Legends: Arceus if the player has save data on their Switch from either Brilliant Diamond or Shining Pearl. Once that's out of the way, you'll also need to beat the entire main story as well.
You'll encounter Darkrai during Request 93: The Darksome Nightmare, which you can find on Laventon's request board after closing the space-time rift.
How to catch Darkrai
Catching Darkrai is either extremely annoying or a walk in the park depending on how kind Darkrai's AI is feeling.
It will not allow you to fight it naturally. You'll need to get the jump on it by using the rocks in the arena to your advantage. Once Darkrai forgets that you exist, sneak around the rock to get behind it and launch one of your Pokémon at it.
Related: How to break the big rocks in Pokémon Legends: Arceus
If you succeed, the rest of the fight will play out as a normal encounter with a level 70 legendary Pokémon. If you failed, our condolences and start again.
You can attempt to catch Darkrai as many times as you need to until you are successful. Even if you defeat Darkrai, the mission will not end until you catch it, so you can return and try again later.
Alternatively, if the sneaking around tactic isn't working for you, you can try to replicate whatever we did below. We aren't sure how we did this or why it happened but it let us skip the sneaking around part, so we didn't look a gift horse in the mouth.
For some help reaching the post-game where Darkrai is, go check out How to solve Azelf’s Trial in Pokémon Legends: Arceus? on Pro Game Guides!
Hello there! My name is Zach Mckinnis, and to sum it up nicely, I am a huge nerd. Games, anime, fantasy - I'm into it all! So to be able to write about one of my passions like this is amazing! Pokémon and Warframe are my jams, but I also play a ton of assorted sandbox and RPG games.