Blizzard have detailed a hotfix that addressed the bugged Battle for Undercity quest by adding a bronze dragonflight NPC that can manually toggle the quest phase, and have also reset the progress for players at a certain stage of the quest line.

Quest Fix (Source)

Greetings,

Just wanted to follow up on this with an update. As of 4:48 p.m. PDT we’ve deployed a hotfix that will add a member of the Bronze Dragonflight “Zidormi” to Tirisfal Glades near the entrance of the Ruins of Loardaeron (for Horde players) or the entrance to the Undercity Sewers (for the Alliance players) that will allow you to manually enable or disable the quest phase that you are placed in for the “The Battle for the Undercity” quest and event.

Horde – Near the entrance to the Ruins of Lordaeron:

Alliance – Near the entrance to Undercity Sewers:

Additionally, when this hotfix was pushed, it will have reset the phase state for all players that are currently on The Battle for the Undercity quest phase. What this means is if you had completed either “Darkness Stirs” (horde) or “Return to Angrathar” (Alliance) or were on any of the ensuing quests up to and including the “The Battle for the Undercity”, you will be restored to the default phase to see the Undercity, Orgrimmar, and the Throne rooms of both Orgrimmar and Stormwind as if you had never started either of those quest chains.

Here’s a full list of the affected quests:

Horde:
• Darkness Stirs
• Herald of War
• A Life without Regret
• The Battle for the Undercity

Alliance:
• Return to Angrathar
• Reborn from the Ashes
• Fate, Up Against Your Will
• A Royal Coup
• Killing Time
• The Battle for the Undercity

Again, if you were actively in-progress for any of these quests, you may need to travel to the Tirisfal Glades and speak with Zidormi to return to the correct phase and resume progress on them either now or at a later time when we are able to implement a full fix for the issues that cause the battle to become stuck. Abandoning and re-accepting these quests will not put you back into the correct phase to see the battle-related events.

This should heal situations where players were “stuck” in the Battle for the Undercity phase and could not interact with vendors or trainers in Orgrimmar or Undercity. The goal with this hotfix was to get players stuck in this phase out of it and skip past this event. We will be doing additional work over the coming days and weeks to further harden the Battle for the Undercity event to make it able to be more consistently completed so that players can get the powerful rewards from this quest more reliably.

We again offer our sincere apologies for these issues, and hope to continue working towards a more complete solution to these issues.