Breastplate of the Lost Vanquisher

Breastplate of the Lost Vanquisher is a tier 7 armor token. It can be exchanged in Dalaran for the following items:
 * Valorous Bonescythe Breastplate
 * Valorous Scourgeborne Battleplate
 * Valorous Scourgeborne Chestguard
 * Valorous Frostfire Robe
 * Valorous Dreamwalker Raiments
 * Valorous Dreamwalker Robe
 * Valorous Dreamwalker Vestments

Source
This item can be found inside the Four Horsemen Chest after defeating the Four Horsemen in the 25-man version of Naxxramas. It can also drop from Gluth.