Adventures
Adventures, or Solo Adventures, are a single-player game mode in which the player participates in matches played against computer-controlled opponents. Adventures can be selected in the Adventure screen, accessed through the Solo Adventures button from the main menu. Adventures offer a unique way to play Hearthstone. Unlike other types of play, each adventure has a specific theme and identity, featuring heroes, cards, art and music in keeping with that setting.
Players must unlock wings of the adventure to discover new bosses and tackle Class Challenges using specially-prepared decks. Adventures can be played in Normal or Heroic mode. The first adventure, Curse of Naxxramas, was released July 22, 2014, the second, Blackrock Mountain, on April 2, 2015, and the third League of Explorers, on November 12, 2015. Cards introduced to the game through adventures are referred to as adventure cards, and are obtainable only through the corresponding adventure. Almost all adventure cards are obtained through defeating specific bosses and Class Challenges.
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You can unlock these adventures either with real money or save up your coins to buy the wings. Certain adventures are better than others because of the cards you receive for completing them; some are used in almost every competitive deck in the ladder.
Curse of Naxxramas
Important cards
Haunted Creeper
Mad Scientist
Zombie Chow
Nerubian Egg
Sludge Belcher
Loatheb
Shade of Naxxramas
Blackrock Mountain
Important cards
Grim Patron
Lots of different dragon cards, which make some popular dragon decks.
The League of Explorers
Important cards
Keeper of Uldaman
Entomb
Tunnel Trogg
Dark Peddler
Reno Jackson
Elise Starseeker
Sir Finley Mrrgglton
Jeweled Scarab
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By far Naxxramas is the most important adventure card set to have. Most of them are still used today. The other two adventures fill niche decks but are super needed.