Festus the Leechlord's experiments to brew a super virus in the sewers of Altdorf. The sewers are described as dark and grimy, Festus is surrounded by human corpses which he collected to provide 'ingredients' and he is regularly visited by Nurgle daemons. In one encounter every corpse in Festus' laboratory turns to face him and says his name at the same time with one voice.
The Skaven go all out against the whole world, destroying Altdorf, Bretonnia, Tilea, Estalia, Karak Azul, Xlanhuapec, Tlaxtlan, Itza and possibly Araby, Cathay and Nippon. The whole of Clan Pestilens invades Lustria, wreaking devastation throughout the continent in ways not seen since the First Great Invasion of Chaos. The Horned Rat has unleashed its servants, the Verminlords, to help its worshippers.
And the reason the Pestilens were able to devastate Lustria so badly? The Grey Seers pulled Morrslieb into a closer orbit, destabilizing the moon, raining boulders of Warpstone into the world. The Slann destroyed most of them, but the sheer strain was too much for them, and the whole race was knocked unconscious.
Clan Skyre proceeds to one-up the Grey Seers in the worst way possible. How? By blowing the moon to pieces with a big gun. Needless to say, if Mazdamundi and Kroak hadn't contained the damage, a massive meteor storm of pure warpstone would've killed everything and everyone on the planet. Even the Chaos Gods themselves were left speechless over Lord Morskittar's actions. The remaining warpstone destroyed Lustria and all of its inhabitants, except for a few Lizardmen led by the returned and undead Kroak, who pulled a Screw This, I'm Outta Here! and fled to outer space.
Duke Jerrod, disgusted over the fact that Lileath and the Lady were one and the same, goes to Bel'Akor and reveals her whole plan to him, most likely dooming Anorath and everyone in the new world to a Fate Worse than Death.
Volume V: Archaon
The worst thing possible happens... Chaos Wins. A third Warp Rift forms in Middenheim, and thanks to Mannfred stabbing Balthazar In the Back, the Incarnates fail to dissipate it, resulting in the whole world being destroyed by a gigantic Warp Storm, killing almost everyone, including the last Bretonnians, the few surviving Tomb Kings, the Halflings, and many others. The volume, and the original Warhammer's story as a whole, ends with Archaon and Sigmar fighting over Ghal-Maraz as they fall into the Warp.
As Age Of Sigmar would later show, by the time the Chaos Gods were done with the World-That-Was, it was a burning, charred husk, stripped of all life and features. Just how strong were those Warp storms?!