Activision Blizzard's (NASDAQ:ATVI) Hearthstone: Heroes of Warcraft is one of the most addictive video games I have ever played, and certainly, the best I have ever played on a tablet. I am not alone in thinking this -- the game, released April 16 for the iPad, currently has a 93 (out of 100) on Metacritic, an aggregator of video game reviews, and ranks among the top 20 free apps on the iTunes chart.

With Heartstone, Activision is set to successfully cash in on two of the biggest trends in video gaming: mobile and free to play -- trends that newcomers, including King Digital (NYSE: KING), have sprung up to take advantage of. If there was any doubt that Activision would be beholden to traditional platforms and methods of distribution, Hearthstone should put those concerns to rest.

Activision finally goes mobile
In the simplest terms, Hearthstone is a digitalized card game: Two players, connected over the Internet, compete against each other using a series of cards played strategically. Matches tend to last 10 minutes or less, and while the rules are simple, the game offers a deep degree of strategy.

Hearthstone is free to download, and completely free to play, but the cards available to players are initially limited. Better, more diverse cards are unlocked over time, but can also be purchased using real money. Right now, the game is only available for the iPad and Windows PCs, but is expected to make its debut on the iPhone and Android devices later this year.

Hearthstone, then, is a unique game for Activision. Of all the major video game publishers, Activision has been the most reluctant to embrace new forms of gaming: It likes its $60 annual Call of Duty releases, and subscription-based World of Warcraft.

Activision's CEO, Bobby Kotick has even at times downplayed and to a degree disparaged mobile gaming. But with Hearthstone, Activision could be sitting on the next mobile gaming hit.

Free-to-play mobile games are swamping everything else
Mobile gaming is growing far faster than the broader industry. According to App Annie, spending on mobile games grew more than 200% in 2012, and on Android, grew more than 400% in the fourth quarter alone.

In the U.S., spending on mobile games totalled $1.78 billion in 2013, according to eMarketer, and research firm Gartner expects mobile games on a global basis to bring in $22 billion in 2015. Much (perhaps most) of that money will go to the creators of free-to-play games.

App Annie notes that in both March and April, the top-grossing app companies -- Supercell, King Digital and GungHo Online -- specialized in free-to-play games like Clash of Clans, Candy Crush Saga, and Farm Heroes Saga. Like Hearthstone, these games generate revenue through the use of optional, in-app purchases.

Hearthstone is a huge opportunity
Right now, the market values King Digital at roughly $5.5 billion -- a bit less than 40% of Activision's market cap. Many have argued that King Digital is grossly overvalued, and perhaps it is, but if Hearthstone could achieve Candy Crush-like success, Activision could reap a tremendous windfall.

Candy Crush generates nearly 80% of King Digital's revenue. Investors are obviously betting on King Digital's ability to create future games that perform as well or better financially, but based purely on its revenue, Candy Crush Saga alone appears to be worth several billion dollars.

Assuming that Hearthstone continues to grow in popularity, Activision investors could get that -- along with Call of Duty, World of Warcraft, and the company's other upcoming PC and console titles.

Rather than focusing strictly on the latest Call of Duty announcement, Activision investors will now have to keep an eye on the app charts.