The Scourge of Lordaeron (History of Warcraft)

After preparing for many long months, Kel'Thuzad and his Cult of the Damned finally struck the first blow by releasing the plague of undeath upon Lordaeron. Uther and his fellow paladins investigated the infected regions in the hope of finding a way to stop the plague. Despite their efforts, the plague continued to spread and threatened to tear the Alliance apart.

As the ranks of the undead swept across Lordaeron, Terenas' only son, Prince Arthas, took up the fight against the Scourge. Arthas succeeded in killing Kel'Thuzad, but even so, the undead ranks swelled with every soldier that fell defending the land. Frustrated and stymied by the seemingly unstoppable enemy, Arthas took increasingly extreme steps to conquer them. Finally Arthas' comrades warned him that he was losing his hold on his humanity.

Arthas' fear and resolve proved to be his ultimate undoing. He tracked the plague's source to Northrend, intending to end its threat forever. Instead, Prince Arthas eventually fell prey to the Lich King's tremendous power. Believing that it would save his people, Arthas took up the cursed runeblade, Frostmourne. Though the sword did grant him unfathomable power, it also stole his soul and transformed him into the greatest of the Lich King's death knights. With his soul cast aside and his sanity shattered, Arthas led the Scourge against his own kingdom. Ultimately, Arthas murdered his own father, King Terenas, and crushed Lordaeron under the Lich King's iron heel.