Undercity is the Undead capital city below the ruins of the City of Lordaeron in east Tirisfal Glades.
WoWScrnShot 022311 125010

Undercity, Tirisfal Glades

The city is spilt up into several districts such as the War Quarter, the Magic Quarter and the Royal Quarter where the banshee queen sits on her throne.

Undercity was first used as a crypt for the dead but when the Arthas invaded he started expanding it into a city until he was called to Northrend. However after he left there was a rebellion and the city was lost to Lady Sylvanas Windrunner and her army.

A few years later just after a battle at Angrathar the Wrathgate the dreadlord Varimathras took over the city with a group of rebels however Thrall lead the Horde forces and killed him but then the Alliance arrived to take the city and King Varian angry at the Horde declared open war then Jaina quicky teleported the King out of the city. After the battle Thrall sent a legion of Kor'kron Guard to Undercity to make sure something like it did not happen again.