You need to complete Legends Arceus first
To get the Azure Flute, Brilliant Diamond and Shining Pearl need to be updated to the 1.3.0 version. Players must also have beaten the Elite Four and received the National Dex. Altogether, getting Arceus in BDSP requires the following:
- Complete all 27 Missions in Pokemon Legends: Arceus.
- Become the Pokemon Champion of Sinnoh and get the National Dex in Pokemon Brilliant Diamond & Shining Pearl.
- Have BDSP updated to 1.3.0 (available March 15, 2022).
The next step is to Fly to Twinleaf Town. Go upstairs to the bedroom and receive the Azure Flute. With this key item in hand, it's time to go to Mt. Coronet and confront Arceus.
It should be noted that the game doesn't require you to have the National Dex to receive the Azure Flute, only to encounter Arceus.
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