Vanquisher's Mark of Sanctification

Vanquisher's Mark of Sanctification is a tier 10 armor token. It can be turned in at various NPCs at Light's Hammer in Icecrown Citadel to upgrade from the base versions of the following tier set pieces:
 * Shadowblade's Battlegear
 * Scourgelord's Battlegear/Scourgelord's Plate
 * Bloodmage's Regalia
 * Lasherweave Battlegear/Lasherweave Garb/Lasherweave Regalia

Source
This item can be found in the Deathbringer's Cache after defeating Deathbringer Saurfang in the 10-man Heroic or 25-man normal version of Icecrown Citadel. It also drops from Professor Putricide, Blood-Queen Lana'thel, and Sindragosa.