“The central plot of the expansion is the return of the evil dragon aspect Deathwing the Destroyer (originally Neltharion the Earth Warder). Last seen in Warcraft II, which took place more than two decades earlier, Deathwing has spent that time healing himself, and plotting his fiery return from the elemental plane of Deepholm. His return tears through the dimensional barrier within Azeroth, causing a sweeping cataclysm that reshapes much of the world’s surface.

In the midst of the worldwide disaster comes renewed conflict between the Alliance and the Horde, which is now under the rule of Garrosh Hellscream. With the elemental realms now open to the world, chaotic elemental spirits and their tyrannical lords emerged to help the Destroyer and the nihilistic Twilight’s Hammer cult bring about the Hour of Twilight: the end of all life on Azeroth.”

- Source: Wikipedia