Blizzard is expanding its Hearthstone options with a single-player campaign, titled Curse of Naxxramas. The studio plans on adding the new adventure mode starting sometime soon, over the course of five weeks. Like the Arena mode, you can buy your way into Naxx with in-game gold or actual dollar-moneys.
Naxxramas comes out of Warcraft lore, as a nightmare-filled necropolis. Visiting in Hearthstone will take you through a series of bosses, and after each wing you'll be able to unlock a chest with new Legendary cards. The Naxx adventure will add 30 new cards in all, and once you've completed the adventure mode you can add them to your regular deck. The campaign will also add a new board and various class challenges.
The first wing, The Arachnid Quarter, will be free, while the other four will require a purchase. A post on Battlenet shows some of the new cards and a look at the boss of the Plague Quarter.