The Enemy Within
The Empire. Perhaps the greatest nation of the Old World, it has stood for two and a half millennia as a bulwark against the threat of Chaos.
Down the ages – from the time of legend when Sigmar Heldenhammer ruled, through the Age of Three Emperors and the Unification under Magnus the Pious, to the crowning of Karl-Franz I and the House of Altdorf, Grand Prince of the Reikland, Emperor, favored of Sigmar, etc, etc – The Empire has endured.
From the Elf-haunted Laurelorn Forest in the north, to the Black Mountains in the south, from the lofty Dwarfholds of the World’s Edge to the creeks and marshes of the Wasteland borders, from Kislev to Tilea, the Emperor’s armies keep the peace. The Empire stands firm against the enemy without.
But what of The Enemy Within?
After that, take a look at your wiki. There is some more helpful info there.