As part of his plan to destroy the Empire, Archaon sends three Nurgle armies to weaken the Empire in preparation for his invasion. The first, led by the Glottkin, a trio of brothers (One a giant mutant, one a sorceror and one a chaos lord), attacks the port city of Marienburg. After a very short battle the city falls, despite the effort of the cities human defenders and a horde of undead led by a Vampire kingpin of the cities underworld. During the battle the Glottkin uses a jar of Nurgles most virulent plagues, which reduces the city to a bubbling mass is hours. Another Empire army from Reikland arrives and almost turns the tide of the battle, but the contagions inside the city undo the Empire's efforts and that army is all but wiped out.
While the Glottkin consolidates its victory, another Chaos Army led by the Chaos Lord Gutrot Spume, joins a large beastman horde and bushes through the Drakwald forest towards the Empire capital of Altdorf. While in the forest, his army is ambushed by a large tribe of forest goblins and savage orcs. A frenzied battle is fought, and Spume is almost killed by an arachnarok spider, but the skill of his heavily armoured warriors ensures a chaos victory.
In Altdorf, the Empire is falling apart. The city is riven by plague, and there are daily riots. The Reikmarshal Kurt Helborg is the de facto leader of the Empire following Karl Franz's disappearance and supposed death. He brutally suppresses the riots, but is blind to the true threat. The nurgle sorceror Festus is in the city and is planning to turn it into a new Garden of Nurgle and as such secure his ascension to daemonhood.
The third Nurgle "army" sent by Archaon is in fact only three warriors. The Daemonkin as they were known, are powerful chaos lords all riding a creature known as a Pox Maggoth. They journey to the Brass Keep in the Empires north. There they summon a vast daemon host and they march to the city of Talabheim. There, after an unsuccessful frontal attack, they use another of Nurgles plague jars to turn the city into a haven for disease. This forces the defenders of the city to face the Daemonkin outside the walls and a short and bloody battle is fought. The city falls and the survivors flee to Altdorf, where Kurt Helborg is preparing the city for an attack.
Vlad von Carstein became aware of this threat, and had sent formal offers of an alliance in order to save the city. Vlad knew that if the Empire fell, then Sylvannia may be next. He also held a strange respect for the Empire (As he did once try to conquer it). After refusing the summons several times Helborg finally accepts after hearing of the fall of Talabheim, and Vlad von Carstein is made an Elector Count of the Empire. Meanwhile in Bretonnia, the civil war that had been taking place had ended when the kings traitorous son was slain by the Green Knight, who is revealed to be the first king of Bretonnia Giles le Breton. Giles is re-crowned king and the former king Louen Leoncoeur, who is now the High Paladin of Bretonnia gathers the largest Errantry Crusade since the war with Arabay. He decrees that any Bretonnian noble who looks to their own fortunes while the Empire fell was not worthy of the title. Only the Grail Knights remain behind. After several weeks of travel the crusade arrives at Altdorf just as the hordes of chaos begin their attack.
The siege of Altdorf begins with the forces of the Empire marching out of the city to form defensive lines around key gates. The Bretonnian army assembles on a ridge overlooking the city. From there they see three great chaos armies emerging from the forest. One is a large formation of heavily armoured warriors supported by hordes of beastmen. Another is a great horde of fur clad warriors and marauders. The third is a daemonic host. The Bretonnians struck first, and a colossal cavalry charge crushes many chaos marauder tribes and beastmen warherds before loosing momentum, only a short distance from the Empire lines. Following this, the Empire Knights charge into the beastmen as they form up and scatter many thousands. Chaos Knights and Dragon Ogres assault the main Empire lines, and are held at bay only just. A squadron of Steam Tanks attack the daemon host, and manage to slow its advance.
Meanwhile, in the cities heart, Festus opens a portal to the warp. Nurgle daemons emerge and the city itself begins to turn into a warped forest. The eight colleges of magic fight back, but suffer great losses. Only the temple of Shallya remains untouched, and it falls to Vlad and his undead horde to defend it. Zombies fight Plaguebearers in the streets around, while Vlad engages the leader of the daemons, Kugath a Great Unclean one. He is joined by Louen Leoncoeur who flies in on his Hippogryph. After a savage duel Louen slays the greater daemon by melting him with his divine blood. Louen then duels Festus, but is beheaded, as the sorceror is too powerful. He is almost granted daemonhood, but Vlad intervenes. Festus seemingly destroys Vlad with his magic, leaving only a ring where the vampire stood. However shortly after Vlad comes back to life and stakes Festus in the heart, killing him. This act saves the city and the daemonic host is banished.
However, outside the walls of the city the Glottkin and its army is on the verge of victory. When it seems the Empire army will collapse Karl Franz is spotted and hope of victory is restored. The battle moves into the city streets and the Empire army tries to hold the chaos armies away from the Imperial Palace. Karl Franz duels the Glottkin in the palace square, and is almost killed. His arm is cut off, but his life is saved by the sacrifice of Kurt Helborg. When all seems lost, Karl Franz calls out to Sigmar for aid. And when he does, the Twin-tailed Comet slams into the city and its energy fills Karl Franz, transforming him into a being of light and lightning. Using this new power Franz slays the Glottkin, and following this the gods Ulric, Shallya and the Lady use their power to heal the god Taal. This in turn heals the Empire from Nurgles taint. The survivors of the chaos army retreat into the forest. Almost half the population of the Empire is already dead, and many of the Bretonnian Knights that rode to its aid have been killed. It is then that news arrives that another Chaos Fleet has been spotted, the largest ever seen and bearing the banner of Archaon.