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.