Chestguard of Undead Slaying

Source
This item was obtained during the Scourge Invasion, the opening event of Naxxramas just after patch 1.11 and is no longer available. It was obtained by gathering 8 Necrotic Runes to interrupt the channeling of the Cultist Engineers near the invasion points. When interrupted, the Acolyte will change into a Shadow of Doom, which would drop the chestguards of the undead slaying sets.