Part of the Vestments of Faith set:

The set gives these bonuses as more pieces are worn: 2: Reduces the mana cost of your Renew spell by 12%. 4: On Greater Heal critical hits, your target will gain Armor of Faith, absorbing up to 500 damage. 6: Reduces the threat from your healing spells. 8: Each spell you cast can trigger an Epiphany, increasing your mana regeneration by 24 for 30 sec. Set Details | All Sets

Quest Reward from:

Level	Name	Start	Rewards
60	Leggings of Faith  (Raid) Horde and Alliance Naxxramas	Father Inigo Montoy <Brotherhood of the Light> Eastern Plaguelands

Disenchants into

Name	Value	Drop%
Nexus Crystal	1 Copper	100.0%