What is this? Is it part of Hearthstone? I haven't played that in ages.
Hearthstone is a competitive CCG but last years they added solo adventures/missions.
At first they were paid adventures (I rember buying Nexxaramss and Kardazan or how they are spelled) and they played like the main game...ie you had to pre build your deck before adventuring against fixed bosses.
My personal problem with Hearthstone, or CCG in general, is that there are way too many cards now and starting building a deck is a daunting task.
I'm an avid card game player but deckbuilders are more my thing (over CCG).
Last year they started adding more solo adventures that play like typical deckbuilders (incidentally after they hired Dream Quest dev).
Those are also completely free.
You start with a super small prebuilt deck and after defeating the story bosses you get to choose which new 3 cards to add to your deck.
Plays a bit like Meteorfall, Nigh of the Full Moon, Dream Quest etc...
If you like those I really recommend you to try out Catacombs and Cobolds or Mirkwood.
On top of that, Blizzard production value is unbeatable.