Battle for the Undercity

Prelude to the Storm
Shortly after the battle for Angrathar the Wrathgate begins and Dranosh Saurfang is slain by the Lich King, Grand Apothecary Putress appears on a cliff overlooking the battlefield and uses Forsaken catapults to unleash the newly developed plague on the unsuspecting forces gathered below. The losses on all sides are great, and perhaps the greatest loss is the death of the Alliance hero Highlord Bolvar Fordragon.

Soon after, it is revealed that Putress and several members of the Royal Apothecary Society had sided with Varimathras, who had secretly remained loyal to his nathrezim brothers and the Burning Legion while serving under Sylvanas Windrunner. Together, the two had orchestrated a coup d'état and forcefully taken control of the Undercity, nearly killing Sylvanas in the process.

Horde
Determined not to allow the Dreadlords a foothold in Horde territory, Thrall and Sylvanas planned an immediate counter-attack. In the midst of their planning, Jaina Proudmoore arrived bearing terrible news: in the wake of Bolvar Fordragon's death, Varian had ordered an assault on Undercity to bring Bolvar's murderer Grand Apothecary Putress to justice and to retake Lordaeron for the Alliance.

After informing that only the Lich King gain with the Horde and Alliance fighting one another. Thrall along with Sylvanas and Vol'jin led the assault into Undercity itself to reclaim it for the Horde. After fighting the Forsaken rebels and the demons within, the combined might of the Horde slew the traitorous demon Varimathras.

Alliance
After receiving word of Bolvar's death Varian is overwhelmed with anger and sorrow, having considered Bolvar a brother ruling his kingdom and raising his son for years. He sent Jaina Proudmoore to Orgrimmar to discover the reason for the Horde's betrayal. He deemed this as an act of treason and demanded retribution for his friend's death. Jaina returned and informed Varian that Varimathras and the Royal Apothecary Society had betrayed them all and the Horde no longer controlled Undercity.

Varian and Jaina gathered the Alliance army and attacked Undercity, hoping to reclaim it as Lordaeron for the Alliance and bring Putress to justice. When entering Undercity, Varian becomes disgusted by the condition of the once great city, which he knew since childhood. Tracking Putress to the Apothecarium, Varian and Jaina defeated him. But, to Varian's horror, they also discovered dozens of mutilated and defiled human corpses, on which the Apothecary Society experimented to create the New Plague; realizing that although the Alliance and Horde agreed to a small truce over the years, the Forsaken had been secretly creating a means to kill them all.

Aftermath
Ultimately, both Varimathras and Putress were killed, and Varian confronted Thrall and Sylvanas in the Royal Quarter. Battle ensued, though Lady Jaina Proudmoore intervened by freezing the combatants solid and teleporting all Alliance forces out.

The Battle for the Undercity ended the cold-war between the Horde and the Alliance, with King Varian's declaration of war.

Meanwhile, in order to ensure that no more Legion-aligned abominations crawled out of the Apothecarium, Thrall sent a legion of Kor'kron Guard led by Bragor Bloodfist to occupy the Undercity, guarding it from threats outside, and from any that may remain within following The Battle for the Undercity.