Hearthstone's next expansion, Whispers of the Old Gods, looks to be fairly simple. No new tribals or keywords, just plain cards that interact with each other and the Old Gods cards. But Blizzard has taken the wraps off of a new spell type with a unique hook of its own.
The Hearthstone Blog has announced "Forbidden" spells, which consume all of your mana for a given turn but give more powerful effects the more mana you're willing to sacrifice. The two examples shown are Forbidden Flame (Mage) and Forbidden Shaping (Priest). The Flame deals damage to a minion equal to the mana spent, while Shaping summons a random minion of the cost of the mana spent.
Forbidden Flame isn't very efficient compared to other damage-dealing Mage spells, but it does put one more tool in the toolbox for when you find yourself in a pinch. Meanwhile Forbidden Shaping may very well become a Priest staple, as finding on-curve minions is something the class traditionally struggles with. We're sure to see more Forbidden spells as the hundred-plus other cards in the set are revealed.
Steve Watts posted a new article, Hearthstone Old Gods expansion to introduce 'Forbidden' cards
Loving all the hype and reveals so far. I primarily play priest casually, and I love the curve filling angle.