In the wake of the Sunwell's purification, a period of suspicious silence swept over the world. As if on cue, the undead Scourge launched a massive assault against the cities and towns of Azeroth, this time extending its reach far beyond the Eastern Kingdoms. Under pressure to respond with a full army, Warchief Thrall deployed an expedition force to Northrend led by Overlord Garrosh Hellscream. Meanwhile, the missing human king Varian Wrynn at last returned to Stormwind City and reclaimed his crown. He sent an equally powerful Alliance army, commanded by Bolvar Fordragon, to defeat the Lich King—and any Horde forces who would stand in their way.
The Alliance and Horde (led by Dranosh Saurfang) eventually led a combined offensive on the Wrathgate, the entrance to the Lich King's fortress of Icecrown Citadel. Before they could succeed, however, Grand Apothecary Putress and his Royal Apothecary Society followers (renegade Forsaken) unleashed a new plague that killed friend and foe alike, while his traitorous counterpart, the dreadlord Varimathras, seized the Undercity in a coup that nearly killed Sylvanas. The usurpers were slain for their vile deeds by armies of the Alliance and Horde and the Forsaken capital was restored. The debacle, however, created suspicion among the Horde regarding Sylvanas's loyalties. At the Wrathgate, many brave Alliance soldiers died at the hands of the Forsaken's Royal Apothecary Society; including King Varian's dear friend Bolvar Fordragon. Varian, who had always been wary of the orcs, discovered that the Royal Apothecary Society had been developing the new plague for years. The events that transpired during the battle for the Undercity convinced the human king that the Horde had been left unchecked for too long and he becomes hostile to the Horde for the rest of the campaign in Northrend.
In order to diminish the leadership of the Scourge, forces of Azeroth take on the newly rebuilt necropolis Naxxramas that floats in the eastern portion of the Dragonblight region. Much like its predecessor in the Eastern Plaguelands of the Eastern Kingdoms, battles wage all throughout the dread citadel, before taking the fight to Kel'Thuzad himself and putting an end to his and the Lich King's partnership once and for all. At the same time, Malygos, Aspect of the Blue Dragonflight, has been twisted and gone mad, convinced the mortal magic wielders are tainting Azeroth and begins a campaign to purge them. Hidden in the void of the Eye of Eternity, he channels laylines of arcane magic to fuel his army and empower his abilities. At the saddened behest of his sister, Alexatrasza, Queen of the Dragons and Aspect of the Red Dragonflight, champions of Azeroth end his crusade and pave the way for a brighter future for the Blue Flight.