Coming out on April 26 across North America (for PC, Mac, iOS and Android devices) and on April 27th for Europe and Asia, Whispers of the Old Gods features 134 new cards for Hearthstone. Hearthstone’s third expansion will be available to purchase via the in-game shop. As the official article says, you can pre-order the game before it is released to receive 50 packs of cards as a bonus. The promotion ends as soon as the game goes live.
Here is a trailer, Whispers of the Old Gods Cinematic Trailer, about the upcoming x-pack: