Undercity

History
The Undercity was originally simply a system of sewers and catacombs beneath the Capital City of Lordaeron. After the city was destroyed by the Scourge, Arthas (who has terrorized the Forsaken) had the underground warren expanded and rebuilt. He originally intended for the Undercity to be his base 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.

http://www.wowwiki.com/Undercity