Shadow Mantle of the Dawn

Shadow Mantle of the Dawn is a special enchantment for shoulder armor that adds shadow resistance.

Source
Shadow Mantle of the Dawn is a quest reward from the quest which starts with Quartermaster Lightspark in Chillwind Camp in Western Plaguelands (for Alliance), Quartermaster Hasana in the Bulwark in Western Plaguelands (for Horde), and Quartermaster Breechlock in Light's Hope Chapel in Eastern Plaguelands (for either). To complete the quest, you must turn the following items to any of those NPCs:
 * x10 Argent Dawn Valor Token

You must also have Revered reputation with the Argent Dawn.

Shadow Mantle of the Dawn is sold by the above NPCs when you complete the quest.