The Mythical Pokémon Arceus and Darkrai are coming to Mythical Pokémon in Gen IV, which means that they require an outside method to acquire them, such as Mystery Gifts or as a save bonus.
In Pokémon Diamond and Pearl, Darkrai could only be encountered on Newmoon Island. The player needed to acquire a Member Card by attending a real-life event or through online distribution, which would give them access to a locked inn in Canalave City. If they slept in the inn, then they were transported to Newmoon Island, where they had the chance to capture a Darkrai. Arceus was meant to have a similar event in Pokémon Diamond and Pearl, but it was never distributed, and players were just given Arceus at events. The original Arceus event involved an item called the Azure Flute, which created a stairway into the heavens, leading to an encounter with Arceus. Pokémon Diamond and Pearl cut the Arceus event, but the Azure Flute item made its way into Pokémon Legends: Arceus as the instrument the player uses to call ridable Pokémon.
It seems that The Pokémon Company is finally ready to give fans the cut Azure Flute event, as the official Pokémon Diamond and Pearl's cut Azure Flute event.
Darkrai will be distributed to Pokémon Brilliant Diamond and Shining Pearl players via a Mystery Gift item. The Member Card will be sent out as a Mystery Gift and it will allow the player to travel to Newmoon Island. It's unclear when the Member Card will be distributed, as The Pokémon Company has yet to send out an official date. Arceus will be available after a new update that is set to launch later today, March 15, so the Member Card might also be distributed soon.
Pokémon Brilliant Diamond and Shining Pearl has already received Mythical Pokémon via s, as it's possible to acquire Jirachi and Mew through save bonuses acquired from having played the other Pokémon games on Nintendo Switch. Shaymin is also a Mystery Gift Pokémon, who can be acquired right now by using Oak's Letter. It seems that all of the Gen IV Mythical Pokémon are returning to Pokémon Brilliant Diamond and Shining Pearl and back into the hands of players.
Source: Pokémon/Twitter, Pokémon
