More than 20 million people have played Hearthstone: Heroes of Warcraft, Blizzard has announced.

The publisher made the announcement over Twitter. "Thank you all for joining us in Hearthstone," said Blizzard. "We're going to need a bigger tavern!"

Hearthstone came out of beta on PC and Mac in March. The free-to-play collectible card game is also currently available on iPad.

Hearthstone is coming to Windows 8 tablets, iOS phones, and Android devices later this year.

In July, Blizzard released the first singleplayer adventure for Hearthstone, called the Curse of Naxxramas. The full expansion adds 30 new cards to the game that are earned by defeating bosses, with completion of each wing guaranteeing the player a new Legendary class card.

Curse of Naxxramas has its own unique game board, as well as nine class challenges that require players to duel using a predetermined deck.