While Antonidas investigated ways to stop the plague, Arthas had fallen into madness, razing Stratholme and traveling to Northrend to hunt the Dreadlord Mal'Ganis. Before the aging archmage could find a cure, Arthas, accompanied by the Lich Kel'Thuzad and an undead army, marched south through Tirisfal and Lordaeron, destroying everything in their path and fueling the war machine with their wake.
Antonidas stood before Arthas and his undead army, refusing to give up the book or the city. Using his unparalleled skills of defensive magic, Antonidas set up a magical field that would destroy any undead that touched it (an earlier version of the defensive shields used today).
Sadly, this was not enough, and Arthas found a way to dispel the enchantment. He marched straight into Dalaran and murdered Antonidas in cold blood. Kel'Thuzad and Arthas entered the Violet Citadel, took the Book of Medivh, and summoned Archimonde.
As the undead army left the city in search of new targets, Archimonde demolished its structures, bringing its magical spires crashing to the ground. Antonidas' Dalaran, now leaderless and in shambles, was no more.
Having fallen in battle defending the only home he knew, Antonidas' legacy continues to be an example for those who live on. His accomplishments, valor, and memory are a testament to the survival of the old world and its new legacy. In this world of constant war, the fragile bonds of a new alliance are but a tribute to the memory and fortitude of one of Lordaeron's bravest sons and greatest heroes.