This set includes new dungeons, cards, a Muse, and a Random Event related to Gal Guardians: Servants of the Dark. Clear the first new dungeon and you'll be able to use the new cards and Muse, as well as trigger the Random Event, in other dungeons.

More details on the official game page of Card-en-Ciel DLC - The World of "Gal Guardians Servants of the Dark" ...