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.
Get stuck in the Undercity No one offering to teleport you out The streets too confusing Don't worry, you can make it to the familiar streets of Orgrimmar in man ways!
Determine how deep into the city you are. Take note that your destination is not in fact inside the Undercity itself, but a little ways outside.
Leave the city. Being that the Undercity is essentially a sewer that the Forsaken faction has Squatted, the way to Orgrimmar would have to be outside the city. This can be achieved one of two ways. You can either leave the city on foot, by finding the convenient albeit hidden elevator leading from the Undercity to the upper level, or if you have a little bit of cash, you can fly to brill, and that will take you to the next town over, very conveniently located near the Zeppelin Towers.
Go to the Zeppelin towers. The zeppelin towers have zeppelins that shuttle back and forth to various locations in the vast Horde territories. The Zeppelins of this the Undercity station are: Grom'Gol, Orgrimmar, and Vengeance Landing. For Orgrimmar, go to the platform at the west edge of the tower. It should have several Horde emblems adorned on the signs. If you are unsure, the goblin at the platform will let you know the Zeppelin's destination.
Board the Zeppelin. do know that you may have to wait a little bit for the zeppelin to appear, for the zeppelin has to shuttle players back and forth between the two cities. After a brief wait and a little bit of loading, You will arrive in the great Horde capitol of Orgrimmar!