
Arthas met heavy resistance from the elves, rallied by Ranger-General Sylvanas Windrunner. Driving their forces before his undead army, he steadily pushed her people back in a swath of destruction towards Silvermoon. Sylvanas tried to warn the vast elven capital of the coming of the Scourge, but Arthas destroyed her camps and killed the Ranger-General. To make her pay for her perceived insolence towards him, Arthas corrupted her elven spirit, transforming it into a horrendous twisted form (a banshee) and enslaving her to the will of the Lich King, forcing her to slay her own people.
Arthas, along with the marching armies of the Scourge annihilated Silvermoon, leaving it in ruins. On the road to the Sunwell, he faced Anasterian Sunstrider, the aged High King of Quel'Thalas, and killed him with minimal struggle. Arthas then used the Sunwell to bring Kel'Thuzad back to life, reborn as an undead lich.
Destruction of Dalaran
As the pair travelled to Alterac, Kel'Thuzad explained the full extent of the "Second Invasion" and the plans of the Lich King and the Scourge. Kel'Thuzad was in Alterac to destroy a camp of Blackrock Clan orcs who had taken control of a demon gate, which the lich would use to speak to the demon lord Archimonde. The Scourge destroyed the orcs, and after Kel'Thuzad received orders from Archimonde, they set out for the powerful seat of the world's mages, the city of Dalaran. Archimonde instructed them to acquire the spellbook of Medivh, which would allow Kel'Thuzad to summon Archimonde into Azeroth.
Despite the Kirin Tor's valiant efforts to repel the invasion, the Scourge fought through their magical defenses and fortifications, killed the Archmage Antonidas and claimed the Book of Medivh.
Arthas and his troops repelled the mages' vast counterattack as Kel'Thuzad began the lengthy summoning of the demon lord. Once Archimonde arrived, he proclaimed that the Lich King was of no further use to the Legion, and Tichondrius was placed in command of the Scourge. Arthas was left to wonder what would become of him and Kel'Thuzad, but the lich replied that all was going as the Lich King foresaw. The pair disappeared as Archimonde took his revenge on the city, destroying Dalaran with a single powerful ritual.