From now until June 4th, Gwent is hosting what they’re calling the “faction premium keg week,” but I prefer to call it the kegapalooza.
Basically, for each keg opened this week, one of the Monsters or Skellige cards will be upgraded to its premium version. These two factions are the last in the line of Faction challenges.
CD Projekt Red puts it as such:
In every keg, each 5th card that belongs to Monsters (meaning every Monsters card that you will see in the “Choose 1 out of 3” part of the keg opening) will be upgraded to its Premium version.
In every keg, at least 1 card out of the initial choice of 4 cards will belong to Skellige.