Arcane Mantle of the Dawn

Arcane Mantle of the Dawn is a special enchantment for shoulder armor that adds arcane resistance.

Source
Arcane Mantle of the Dawn is a quest reward from the quest Mantles of the Dawn 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:
 * 10x Argent Dawn Valor Token

You must also have Revered reputation with the Argent Dawn.

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