Heart of Darkness

Heart of Darkness is a reagent for many blacksmithing, leatherworking, and tailoring recipes found in the Black Temple.

Source
This item drops from trash mobs in the raid instances Black Temple and (as of 2.2) Hyjal Summit.

Blacksmithing

 * Dawnsteel Bracers
 * Dawnsteel Shoulders
 * Shadesteel Bracers
 * Shadesteel Girdle
 * Shadesteel Greaves
 * Shadesteel Sabots
 * Swiftsteel Bracers
 * Swiftsteel Shoulders

Leatherworking

 * Bindings of Lightning Reflexes
 * Boots of Shackled Souls
 * Bracers of Renewed Life
 * Bracers of Shackled Souls
 * Living Earth Shoulders
 * Living Earth Bindings
 * Greaves of Shackled Souls
 * Redeemed Soul Cinch
 * Redeemed Soul Legguards
 * Redeemed Soul Moccasins
 * Redeemed Soul Wristguards
 * Shoulderpads of Renewed Life
 * Shoulders of Lightning Reflexes
 * Swiftstrike Bracers
 * Swiftstrike Shoulders
 * Waistguard of Shackled Souls

Tailoring

 * Bracers of Nimble Thought
 * Mantle of Nimble Thought
 * Night's End
 * Soulguard Bracers
 * Soulguard Girdle
 * Soulguard Leggings
 * Soulguard Slippers
 * Swiftheal Mantle
 * Swiftheal Wraps