This is thanks to a Recuit A Friend function which is now in the game. Using said function to recruit a friend will reward you with Morgl the Oracle, a Murloc Shaman as a playable hero.
It's not actually as simple as forcing a friend to sign up so you can listen to an amphibious wizard burble as you play. NO. That way would lead to a sudden increase in throwaway accounts that never get played and are linked to email addresses Murloc fanciers had set up for this very purpose.
Instead Morgl will only unlock when your first recruited friend amasses 20 levels across their hero roster. Or when you've levelled the account linked to email@example.com to 20 across your second set of heroes and can finally go back to your main. Whichever. No judgement.
There are a host of other tweaks and changes which you can check out in the full patch announcement. I must say they don't really mean much to me what with my giving up Hearthstone relatively early on but I do like the iOS bugfix: "Hearthstone will no longer crash when the device is in a certain orientation when the game is launched."
They should have called it the Zoolander patch.