Mario Kart is a blast and the Destiny beta is everywhere, but the one multiplayer game that's made the most impact on me is Hearthstone, a video game about the power of zoos.

Really, Blizzard's digital card game is one of the best games of the year—it's addictive, accessible, and full of moments that seem designed just to get the endorphins rushing (like that gold smoke that pops up in the card-opening menu just when you're about to get a sweet new Legendary).

And now, thanks to Curse of Naxxramas--which launched this week and will continue expanding through August—Hearthstone's got a single-player mode. Blizzard's multiplayer game now has its very own campaign.

So what does that mean, exactly? For starters, don't expect a story—Hearthstone is still all about card battling, and unlike Blizzard's other big games, this one doesn't even bother with emotional moments or an intricate plot. The shtick is this: you're going to fight a bunch of spiders with cards. That's it.

Single-player in the world of Hearthstone means doing the same thing you do in multiplayer—casting spells and summoning minions to take down your opponent's spells and minions—but instead of playing against another person, you're playing against a computer.

Now what's cool about Naxxramas, which I've been playing around with this week, is that the AI opponents are totally overpowered. The first wing of Naxx includes three bosses, all of whom have rigged special powers (like a three-mana ability that sends your minions back to your hand) and crazy new cards. So instead of using the decks you might use to take on human-controlled mages, rogues, and priests, you've gotta come up with new strategies, most of which involve trying to control an insane number of ever-respawning spiders.

The whole thing is relatively easy at first, but once you defeat all three bosses on Normal mode, you get to beat them all again on Heroic mode, which is like Hearthstone on speed. Since Hearthstone is already digital crack, this is probably a dangerous combination of drugs that should not be consumed by anyone.

I won't spoil much about the bosses and the strategies you'll need to defeat them, because figuring it out on your own is part of the fun, but you've gotta put yourself in a totally different head-space than you'd use for standard battling or even Arena mode, and that in itself is enough of a reason to download Naxxramas and test it out. (Plus there's the whole "get a crapload of overpowered new cards" thing.)

If there's one complaint I have about Naxx—other than the lack of rewards in Heroic mode—it's that there's something a little bit off about the tone. The bosses are all deadly serious, threatening your life with hisses and insults, but the ever-present lich Kel'Thuzad—a tough boss in World of Warcraft—comes off more wise-cracking blogger than bone-chilling wizard. Some of his comments—"Sea Giant?? Maexxna. that is not on my approved card list!"—are hilarious, but also sort of... dissonant.

Still, this game is a blast. Over the next four weeks, Naxxramas will continue opening up new wings, and while the first section is totally free—you can get it now on your computer or iPad, no catch—the other four will be $7/pop (or $20 as a bundle). If you're into Hearthstone, I can't see why you wouldn't.