Far beneath the palace of Lordaeron, the Undercity held the ancient crypts and catacombs of Lordaeron's royalty, as well as the city's dungeons and sewers.
In the Third War, Prince Arthas slew his father and ransacked the city. He planned to use Lordaeron as his capital in the Eastern Kingdoms and ordered his minions to expand the catacombs beneath the city. Then the Lich King summoned Arthas to Northrend and the work was left uncompleted.
During Arthas' leave, Sylvanas Windrunner broke free of the Lich King’s control, taking banshees and other undead with her. With Varimathras' assistance she defeated the dreadlords that controlled the city and set her newly dubbed Forsaken to finish Arthas' job.