From the official site:

An ancient evil lies dormant within Deepholm, the domain of earth in the Elemental Plane.

Hidden away in a secluded sanctuary, the corrupted Dragon Aspect Deathwing has waited, recovering from the wounds of his last battle against Azeroth and biding his time until he can reforge the world in molten fire.

Soon, Deathwing the Destroyer will return to Azeroth, and his eruption from Deepholm will sunder the world, leaving a festering wound across the continents.

As the Horde and Alliance race to the epicenter of the cataclysm, the kingdoms of Azeroth will witness seismic shifts in power, the kindling of a war of the elements, and the emergence of unlikely heroes who will rise up to protect their scarred and broken world from utter devastation.