The Ebon Blade Quartermaster Duchess Mynx may be found at the Shadow Vault—building found at the northwestern corner of the Icecrown Glacier region. However, you won’t see the Ebon Blade NPCs there. Blizzard uses phasing quest system technology to unlock it. Talk to Koltiras (death knight) at the Orgrim’s Hammer (The Horde gunship) or Thassarian (Alliance gunship). They start a quest to infiltrate and turn to the Ebon Blade’s cause three Scourge NPCs who will help you later to kill the Shadow Vault leader General Lightsbane. Once you have killed General Lightsbane, return to the Gunship. The death knight will ask you to return to Shadow Vault. When you return, the Shadow Vault is no longer the way you saw it before. No Scourge NPCs are in the area. Instead the Ebon Blade NPCs have made Shadow Vault your new Inn.
You will find the Quartermaster inside the building, to the west. Wearing the Ebon Blade tabard gives you extra reputation gain in any level 80 dungeon run.