The Master Ball is a unique Poké Ball in Pokémon Sword and Shield. When you toss it, you’ll have a 100 percent chance of capturing whatever Pokémon you hit with it. It’s exceptionally rare, and because of it always catching a Pokémon, trainers seek after it to ensure they can at all times obtain the rarest encounters.
To get it in Pokémon Sword and Shield, you need to make your way through the entire game. You don’t beat the game merely by acquiring all of the eight gym badges. You complete it by going through and taking on Battle Tower, where you need to face Leon, the helpful Pokémon Champion who you’ve seen throughout the story. Take him on, win, and enter the hall of fame.
After that, you return to your home town and back to your house. Eventually, Professor Magnolia should come to your home, and she’ll want to speak to you. She congratulates you on your success in completing the Battle Tower and becoming the new Pokémon Champion.
She’ll hand you the Master Ball shortly after. Because you have to return to your home at the end of the game and speak to her, you cannot miss it.
You can obtain another Master Ball by winning it through the Loto-ID. You can earn a Master Ball after matching five or more numbers through the ID system. To ensure you never miss a chance to win the Loto-ID, check back in at the computer in the Pokémon Center to see if you won that day. To increase your odds of winning, go out there and catch as many Pokémon as you can.