The Undercity is the capital city of the Forsaken undead of the Horde. It is located in Tirisfal Glades, at the northern edge of the Eastern Kingdoms.
Far beneath the ruined Capital City of Lordaeron, the royal crypts have been turned into a bastion of evil and undeath. Originally intended by Prince Arthas to be the Scourge's seat of power, the budding "Undercity" was abandoned when Arthas was recalled to aid the Lich King in distant Northrend. In Arthas's absence, the Dark Lady, Sylvanas Windrunner, led the rebel Forsaken to the Undercity, and claimed it for her own. Since taking up residence, the Forsaken have worked to complete the Undercity's construction by dredging the twisted maze of catacombs, tombs, and dungeons that Arthas began.
Where is the mount vendor in Undercity
It's actually located in a town outside of the Undercity, in Brill. You can arrive at Brill by taking the road North out of the Ruins of Lordaeron above Undercity.
Captain Donald Adams, in the center of Undercity, sells Undercity Tabard which grants reputation in all dungeons.
Large amounts of Undercity reputation can be gained in the following zones: Arathi Highlands, Hillsbrad Foothills, Ruins of Gilneas, Silverpine Forest, The Hinterlands, Tirisfal Glades, Western Plaguelands. There are also daily quests available from the Argent Tournament, as well as Undercity Writ of Commendation from the Molten Front.
Non-Forsaken Horde players can purchase up to seven skeletal mounts from Zachariah Post in Brill--with Exalted reputation.
Notes, tips, and additional info
There are two Zeppelin towers within fairly short walking distance of the Ruins of Lordaeron, one offers flights to Orgrimmar, Durotar and Grom'Gol Base Camp in Northern Stranglethorn, the other to Vengeance Landing in the Howling Fjord in Northrend.
Do not worry about falling into the green liquid; it is harmless, and you may get out of it by finding a sewer pipe exit (exterior of the Undercity) or a small stairway (interior of the Undercity). You can even fish on it, but you won't catch anything magnificent....
The Undercity doesn't have a fruit vendor.
Using [Detect Invisibility] or a similar ability within the Ruins of Lordaeron (within the walls but before entering the throne room), it is possible to see low-level undead mobs (each named "Lordaeron Citizen"), similar to the Unseen in Duskwood.
Though from the outside players see many towers and rooftops, they are in fact nothing more than an illusion. The former Lordaeron capital is sparsely decorated in-game with only the courtyard, translocater room and tomb being an actual part of the former structure. The game designers did not intend for anyone to see Lordaeron from above, or get to other parts, just to walk through it. This was changed in World of Warcraft: Cataclysm, and the ruins of Lordaeron are now fully built.
Balconies above the inner circle parts of the War Quarter, Mage Quarter, Rogues' Quarter, and Apothecarium can be seen while flying in and out of the city by taxi. These empty balconies, aside from a single food crate, are on the same level as the Trade Quarter and are connected by footbridges that span above the corridors leading to and from the Trade Quarter and the outer quarters. These balconies and their connecting bridges can actually be seen on the map of Undercity. They cannot be reached on foot, but a mage can use [Slow Fall], a priest can use [Levitate], and an Engineer can use a [Parachute Cloak] to reach these balconies. From the top of the sewer tunnel where vampire batspass, a player can activate either any one of these, mount up, run and jump, aiming to the left of the tunnel, to land on the balconies above theWar Quarter. Video of how to reach them.
There are portions of the Undercity where effects from Warcraft III were deliberately inserted into the game as ambient sounds. These areas are the throne room, the hallway directly outside of it, and even the bell tower located just at its entrance. Near the bell, you will hear the churchbells ringing when Arthas returns to Lordaeron before he becomes a death knight.
In the hall before the throne room, there are still scattered rose petals on the cobblestones from Arthas' return. In the throne room itself, a slight trail of blood ¡ª from when Terenas' crown rolled on the floor after Arthas murdered him ¡ª can be seen near the throne. What is most chilling is that the room echoes with Arthas' own voice saying the words, "Succeeding you, father," his words before he murdered his own father.
On the in-game map of the Undercity, the emblem of the Scourge is used instead of the regular Forsaken emblem.
Sylvanas Windrunner seems to find the Undercity appealing to the eye, in spite of its rotting guards, necromantic constructs and mysterious green fluids. When she, Thrall and the player invade the Undercity during the struggle to retake it, she exclaims, "What have they done to my beautiful city "
At one time there was a glitch in the game, so that a character running towards the opposing wall of a descending elevator would fall through the world, into Stonetalon Mountains, and then the Alterac Mountains. This may suggest that Blizzard used a vertical system for zone placement in addition to the existing horizontal one, creating another reason for not allowing flying mounts in classic content.
Guards spawned from civilians are only level 65.
A Tauren riding on a Kodo Mount has to dismount to enter the elevators through the throne room.
A book at Acherus: The Ebon Hold states that "Our sources inside the Undercity report that the Horde is mobilizing to stage an assault on Northrend.", revealing that Undercity have, or had, infiltrators within the city walls.
The new Orcish guards replacing most of the Forsaken guards and Abominations inside Undercity also have their own flavour of text responses when players interact with them when finding out something like Class or Profession trainers. They have distinct reservations about Undercity, from its Inn, to its Cooking and sometimes hostile responses, particularly for players wishing to find the Warlock trainer.