Almost fifteen years later, the Plague of Undeath broke out in Lordaeron. Terenas' son Arthas, a paladin trained by Uther the Lightbringer himself, made efforts to stop the Plague from turning his beloved people into mindless undead raised to serve the Lich King, master of the Scourge. Instead, the whole chain of events appears to have been engineered by the Lich King himself to find a suitable host for his bodiless mind, and Arthas fell into despair and eventually madness. Travelling to Northrend, he forfeited his soul to the Lich King upon taking up the runeblade Frostmourne.
Now the greatest of the Lich King's death knights, Arthas travelled back to his homeland, murdered his father and all but obliterated the Kingdom of Lordaeron. Afterwards, he was tasked by Tichondrius, the leader of the Dreadlords and the Lich King's chief jailor, to resurrect the necromancer Kel'Thuzad - whom Arthas had killed not long before - so he could fulfill his appointed duty of summoning Archimonde into Azeroth. Tichondrius also told Arthas that there was only one suitable place to resurrect Kel'Thuzad - the mystic Sunwell, the source of the high elves' magic, deep inside Quel'Thalas.
To reach the Sunwell, Arthas needed an insider, someone who could allow him to bypass the magical defenses of Quel'Thalas; he found one in Dar'Khan Drathir, a member of the Convocation of Silvermoon. Dar'Khan, an egotistical and embittered man, believed he deserved more than he was receiving. Arthas preyed upon his ego, promising Dar'Khan untold power in exchange for his aid, and his loyalty. With the backing of his "Blessed Lord Arthas", Dar'Khan provided the Scourge with two powerful artifacts - the Stone of Light and the Stone of Flame - that had warding powers against the undead, allowing the Scourge armies to circumvent Quel'Thalas' formidable defenses. Thus, Arthas led the Scourge in a march of death up a path that today is known as the Dead Scar, across the Elrendar River into Eversong Woods, destroying one of the protective runestones on the edges of the forest.
The elves, in an attempt to stop the blight from spreading further into Eversong Woods, set the area around the destroyed runestone to the torch. But the armies of the Scourge pressed on, breaking through the Elfgates and through Silvermoon itself. Ranger General Sylvanas Windrunner led the defense as best she could, harassing Arthas at every opportunity, but she eventually fell to the power of Frostmourne. In a cruel gesture of dominance, Arthas took Sylvanas' lifeless body and ripped her spirit from it, creating the first banshee. Even the blade Quel'Delar, King Anasterian and the Convocation of Silvermoon were no match for Arthas' legions, slaughtered with their people by the relentless tide of the mindless undead.
With Silvermoon in ruins and the elves broken and scattered, Arthas fulfilled his task and submerged the corpse of Kel'Thuzad within the waters of the Sunwell, fouling the potent waters of Eternity; the necromancer returned to the world of the living as a sorcerous lich. Not long afterwards, the Sunwell itself exploded, cutting off the elves from their source of magic.