This item is acquired in-game. See its sources below.
In the Appearances collection, Eavesdropper's Helm is listed as Quest. Appearance ID: 70178. 7 other items wear the same appearance:
Read from the World of Warcraft client build files.
Plate is the armor type Death Knight, Paladin and Warrior wear. With Strength or Intellect, Eavesdropper's Helm suits Arms, Fury and Protection Warrior; Holy, Protection and Retribution Paladin; Blood, Frost and Unholy Death Knight.
Eavesdropper's Helm is part of the transmog set Trailblazer's Plate Armor (Warrior, Paladin and Death Knight only). The other 16 pieces:
Expedition Guard's Sabatons
Expedition Guard's Greaves
Expedition Guard's GauntletsExpedition Guard's Legguards
Expedition Guard's Spaulders
Expedition Guard's Girdle
Expedition Guard's VambracesExpedition Guard's Helm
Expedition Guard's ChestguardRuby Guardian's GirdleExpedition Guard's WristguardsReinforced Beekeeper's HelmArtist's WaistguardSister's KeeperTime WalkersChronomental GauntletsBuilt from the World of Warcraft client build files and recorded Auction House snapshots.
Eavesdropper's Helm is not traded on the Auction House, so it has no Auction House price. It is acquired in-game from the sources listed above. This page collects its first-party game data (identity, effects and where it comes from) from the official World of Warcraft client build.
TheWoWDB