Ring of Blackrock

Ring of Blackrock increases damage and healing done by spells and regenerates mana. This item is best suited to healing classes, and is especially useful to shadow priests because of their lack of innate mana regen (as compared to Mage Armour, Conjured Rubies etc. The mp5 on this ring is most effective over long PvE boss fights, and is of limited use solo, or in pvp

Source
Ring of Blackrock drops off Ebonroc, Firemaw, and Flamegor in Blackwing Lair.