The story so far
Quick recap on what's happened in WoW over the last year or so, after the defeat of Arthas at the end of Wrath of the Lich King, the Alliance and Horde were fairly settled with one another. However, a few months after the end of the war against the Lich King, the corrupted leader of the black dragons, who had been forced into hiding since the end of Warcraft 2, emerged from deep beneath the earth and radically changed the landscape of Azeroth as well as breaking a load of other shit in the process.
Prior to this, the Horde warchief, Thrall, sensed that there was something drastically wrong and stepped down from his position to further train as a shaman. Leaving Garrosh Hellscream, the son of Grom Hellscream (Who helped defeat Mannaroth in Warcraft 3) to serve as the new warchief. However, under his leadership, many members of the horde, particularly the Trolls, felt that Garrosh's warmongering was causing huge divides within the ranks.
Eventually, Garrosh was challenged by the Tauren Chief, Cairne Bloodhoof, for the right of leadership in a duel to the death. The two eventually dueled and, to the horror of the onlooking Tauren, Garrosh killed Cairne, who was unaware that Garrosh's axe was poisoned by a rogue Tauren priest.
After stopping Mount Hyjal from being burned to ashes by Deathwing's minions and travelling to the realm of earth, Deepholme, to prevent azeroth from collapsing in on itself. The heroes of the Alliance and Horde successfully lead assaults on strongholds that were occupied by Deathwing's followers and allies.
The leaders (also known as Aspects) of the remaining dragons, held a council after it was found that Deathwing's masters, the Old Gods (think Cthulu in the Warcraft universe), were finally playing their cards in an event known as the 'Hour of Twilight', which will kill every living thing on Azeroth if it isn't stopped.