The Undercity was originally simply a system of sewers, crypts, and catacombs beneath the Capital City of Lordaeron. After the city was destroyed by the Scourge, Arthas had the underground warren expanded and rebuilt. He originally intended for the Undercity to be his seat of power, from which he would rule the Plaguelands. However, shortly after the Third War ended, Arthas was forced to return to Northrend and save the Lich King. In his absence, Sylvanas Windrunner and her rebel Undead captured the ruins of the city. Soon after, she discovered the massive underground fortress, and decided to establish it as the main base of operations for the Undead Forsaken.
After the Forsaken were betrayed by Grand Apothecary Putress at Wrathgate, who split off from the Horde to launch a lethal chemical assault, the Forsaken were nevertheless mistrusted by the rest of the Horde. They have tried to demonstrate their loyalty in various ways, such as attacking the kingdom of Gilneas.
In addition, with the death of the Lich King, Sylvanas has accepted members of the Scourge into the Forsaken. Some speculate that she has used the Lich King's methods for her own needs, using the Val'kyr to resurrect bodies, reprogramming them as mindless servants to further the Forsaken's agenda.
Warchief Sylvanas Windrunner
Forgotten Shadow, Holy Light,Demonology
Southern Tirisfal Glades