In a new World of Warcraft trailer, the beating heart (pun fully intended) of the story behind the 5.4 update for World of Warcraft features the invasion by Garrosh into the Veil of Eternal Blossoms, where he kills a Pandaren and dumps the heart of the old god Y’Shaarj into the well, corrupting the Veil. Meanwhile, Ogrimmar is a new battleground for Horde and Alliance alike.
Check out the trailer for the Siege in the video below, as the 5.4 update seems to be almost an expansion unto itself - or maybe a setup for a new expansion.