Hearthstone's most recent update came with quite the surprise for data miners. Along with the single-player "Monster Hunt", a riff on Kobolds & Catacombs' Dungeon Run mode and part of The Witchwood expansion, the new files included loads of never-before-seen cards with ridiculous powers, none of them related to any official set. Where did these cards come from?
According to Hearthstone Top Decks, the cards are part of a so-called "Slush" set. It's important to note this isn't a future set. These cards may never be added to the game and a lot of them would be busted-as in their current form.
It's pretty clear the Slush is where Blizzard tests card ideas. It also looks like it's home to cards that were cut or eventually reworked. For instance, there are a couple of "Genn Greymane" cards in there, even though Greymane is already a playable card in The Witchwood.
If you don't care about seeing them as proper cards, the raw text is also available.
I can't come up with a convincing argument as to why Blizzard would intentionally add this testing set to the game. In other words, someone stuffed up and I wouldn't be surprised if the data was removed in a future update.