He traveled to the city of Lordaeron and took the city back from the dreadlords.

Meanwhile, back on Azeroth, Arthas had travelled to Northrend and by this point had already become the dominant personality of the Lich King. The Blood Elves were betrayed by the Human commander, so the Blood Elf leader, Kael’thas Sunstrider fled to Outland to join Illidan’s Forces. They were also fighting Illidan’s Naga forces (the Naga are former Highbourne Elves). The blood elves made an alliance with the night elves, and were able to push back the overwhelming undead. There were quite a few struggles over the years, and to make a long story short, a new Alliance of Humans and Blood Elves arose, who fought against the Scourge as well as the Legion. Some of the ‘left-over’ soldiers would continue to spread the Legion’s influence, some would just gather simply because they could, and others would gather, claiming to protect sites of power while trying to find a more permanent portal solution. This new council continued expanding some of the legions corruption, mainly through the Satyrs and the twisting of Furbolgs, a bear-like race. There was a new Shadow Council that was formed, however these cultists proved to be no threat to Azeroth, and they were reformed by the surviving warlocks.

Some abandoned the Legion, and some still served loyally. Most of the forces were scattered in Azeroth without any of their leaders. After this defeat, the legion commanders returned to the Twisting Nether to lick their wounds.

Source of the corruption wow legion