From December 14th, 2023 until Wednesday, January 3rd, 2024, at 3:59 p.m. PST, players of Pokémon Scarlet or Pokémon Violet can receive a Master Ball via Mystery Gift.

Add a Master Ball to Your Inventory

Claim a Master Ball via Mystery Gift and catch a wild Pokémon without fail in your Pokémon Scarlet or Pokémon Violet game.

Guarantee that you’ll catch your next treasured partner by claiming a Master Ball via Mystery Gift in Pokémon Scarlet or Pokémon Violet. Not sure which wild Pokémon to add to your team? Celebrate the appearance of Legendary Pokémon from throughout the series in The Hidden Treasure of Area Zero Part 2: The Indigo Disk DLC by using your Master Ball to catch a Legendary Pokémon of your own. Claim your Master Ball by selecting Get via Internet from the Mystery Gift menu in your game by Wednesday, January 3, 2024, at 3:59 p.m. PST.

To encounter certain Legendary Pokémon in The Hidden Treasure of Area Zero Part 2: The Indigo Disk, you’ll need the assistance of Snacksworth, a curious character you can meet after completing the main story of The Indigo Disk. Snacksworth will ask that you complete Blueberry Quests (also known as BBQs), which are extracurricular activities at Blueberry Academy that you can complete in the Terarium. Do as Snacksworth requests, and he’ll offer you opportunities to encounter Legendary Pokémon from throughout the Pokémon series in the Paldea region.

Have fun mastering this new opportunity!

Source: https://www.pokemon.com/us/pokemon-news/add-a-master-ball-to-your-inventory