Hearthstone's next expansion, announced today, is called Knights of the Frozen Throne.
It takes the game's nine heroes, kills them, then recasts them as undead Death Knights, raised from the grave by the Lich King himself:
Each Death Knight was once a mighty hero of Azeroth -- but after the Lich King raises a hero from the dead, they gain unimaginable strength from beyond the grave. In Knights of the Frozen Throne, each of Hearthstone's nine heroes has embraced the Lich King's dark power to serve him as a Legendary Death Knight Hero card! Despite their transformation, they have not fallen completely under the Lich King's sway, so it will be up to you to decide how to wield the awesome abilities at their command.
Hero cards are an entirely new card type that are neither spell, minion, nor weapon. When played, each Death Knight Hero card replaces your hero, casts a Battlecry, dons some hefty armour, AND provides a new Hero Power. While each Death Knight hero is different, they all offer possibilities that can transform the way you play Hearthstone.
Knights of the Frozen Throne is out in "early August".