Crystallized Shadow

Source
Dropped by various shadow themed creatures in Northrend zones, such as:
 * Shadow Revenant
 * Wandering Shadow

Crystallized Shadow can also be mined from Rich Saronite Deposits and Saronite Deposits.

Crystallized Shadow as an ingredient

 * (1 Eternal Shadow = 10 Crystallized Shadow and vice versa)

Alchemy

 * Mighty Nature Protection Potion

Blacksmithing

 * Saronite Ambusher
 * Saronite Shiv

Engineering

 * Iceblade Arrow
 * Mechanized Snow Goggles
 * Mechano-hog (horde)
 * Mekgineer's Chopper (alliance)
 * Nesingwary 4000
 * Visage Liquification Goggles
 * Wormhole Generator: Northrend

Leatherworking

 * Dark Arctic Chestpiece
 * Dark Arctic Leggings
 * Dark Iceborne Chestguard
 * Dark Iceborne Leggings
 * Scaled Icewalkers

Tailoring

 * Duskweave Boots