The Mists Part

A lost continent, Pandaria, emerged from the mists that had hidden it since the Sundering. Its peace guarded a dark secret: the Sha, physical manifestations of negative emotion - doubt, hatred, despair - left behind by an Old God the pandaren had buried long ago.

Warchief Garrosh Hellscream, Thrall's chosen successor, turned the Horde into an engine of conquest, destroyed the city of Theramore with a mana bomb, and tried to weaponize an Old God's heart. His own people and the Alliance stormed Orgrimmar together to depose him - the first crack in the idea that the Horde and Alliance could only ever be enemies.