The Pokemon Company is offering a special introductory offer to Pokemon Global Link members when Pokemon Ultra Sun and Ultra Moon launches next week.
From November 17 to January 30, 2018, trainers who Game Sync (link their Pokemon Global Link account to their copy of Ultra Sun or Ultra Moon) are entitled to receive a Master Ball. If they own two copies of the game, they will get two Master Balls.
The Master Ball will be distributed starting January 2018.
More details on Game Sync can be found here.