Hearthstone just got a bit more single-player content today.

The second part of the Blackrock Mountain add-on for publisher Blizzard’s digital card battler is now live (in the Americas and the EU — everyone else will get it soon). You can take on three new bosses in the Molten Core, which is the name of this wing. As per usual, the standalone price for this episode is $7, but you can get it along with access to all other Blackrock Mountain wings by paying $25 up front. That price will eventually get you access to 31 new cards as long as you go through with beating the boss characters in the adventure.

This new content is one of the major ways that Blizzard generates revenue for Hearthstone. That’s important because the free-to-play game is already one of the company’s biggest successes.

The app is the No. 6 highest-grossing iPad game in South Korea, which is one of the largest mobile markets in the world. It is also in the top 20 in the U.K. Last week, as Blackrock Mountain launched, players rushed the App Store to buy the add-on. And that pushed the game all the way up to No. 12 spot on the overall highest-grossing app chart in the United States. It has since come back down now that most people have just bought the $25 pass, but it is still in the top 30.

From a day-to-day standpoint, Hearthstone makes plenty of money on PC, iOS, and Android from its booster packs and the Arena mode, which people can pay real money to gain entrance to. But add-ons like Blackrock Mountain (and Curse of Naxxramas before it) guarantee a certain amount of new and interesting cards (that you can only get by paying) without having to wait for random drops from buying card packs. That encourages dedicated players — even those who normally wouldn’t spend money in a free-to-play game — to pony up their cash, and it also gets lapsed players to return to see what’s new.

Hearthstone is Blizzard’s first mobile game and first free-to-play release that makes all of its money from in-app purchases, and the publisher has shown that it has a knack for this kind of business.