Hearthstone players can the first wing of the Curse of Naxxramas expansion pack's singleplayer adventure campaign.
The 'Arachnid Quarter' is free to all Hearthstone players during the expansion's launch event, which will end in early September. Time to get working on those card-game skills again.
Additional wings of Naxxramas will slowly be added over the next few weeks. To play those, however, you'll need to either fork out $6.99 or 700 pieces of in-game gold. Another option is to buy one or two wings with in-game gold, then grab the rest for a reduced real-money rate. I've posted the necessary information in the image at the bottom of this post.
Here's something surprising: the Arachnid Quarter's got lots of spiders in it. Most notably the final boss, a massive spider called Maexxna, and Grand Widow Faerlina, the "Mistress of Venom". Defeating these bosses will net you some new spidery cards.
The first expansion pack for Hearthstone, Curse of Naxxramas is based on the World of Warcraft dungeon of Naxxramas, and contains card-battling versions of the same bosses.