Hearthstone adds a Murloc Shaman Hero to the game via Recruit a Friend!
Love Hearthstone but want an extra incentive to get friends involved? Well good news then because now Blizzard has added a recruit a friend system (much like WOW) that allows players to bring a buddy to the game to unlock Morgl the Oracle who is a Shaman Hero that replaces the default Thrall.
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To get Morgl all you need to do is invite a buddy in-game and make them level up to level 20 (it doesn't have to be on one class, can be split among all classes).
In addition if you recruit a buddy you will both receive a classic card pack for free (you can only recruit up to 4 players at present though).
Source: http://uk.ign.com/articles/2016/07/13/new-hearthstone-hero-requires-friends-to-unlock-it