Here's how you get the Azure Flute:
1. Complete the National Pokédex: This is the primary requirement. You need to catch or see all 151 Pokémon in the Sinnoh region.
2. Speak to Professor Rowan: After completing the National Pokédex, go back to Professor Rowan's lab in Twinleaf Town. He will give you the Azure Flute as a reward.
The Azure Flute is used to summon the legendary Pokémon, Shaymin, into the game. It is a very valuable item for trainers looking to complete their collection!