Doomsayer's Robes

The Doomsayer's Robes are a semi-common drop from two NPCs, added in Patch 7.3.5, that can only be found in the opposite faction's capital city.

Source
This item drops from the following NPCs:
 * Charles Gastly in Orgrimmar
 * Gordon MacKellar in Stormwind

The targets will only be lootable if the robes drop (there's no loot otherwise), and will despawn in a matter of seconds. They respawn in roughly 5-7 minutes.