This Crystallized Shadow farming guide will show you where I farm Crystallized Shadow in Wrath of the Lich King Classic.
Eternal Shadow comes from combining 10 Crystallized Shadow, so if you need Eternal Shadow, you have to farm Crystallized Shadow.
Deathbringer Revenants at the Frostmourne Cavern in Dragonblight is the best source of Crystallized Shadow. This cave was designed so that there are always 4 of these mobs up, so when you kill one that would take their numbers below that, a new one will spawn instantly.
You can also farm Wandering Shadows at Wintergrasp. These have a higher drop rate but are more spread apart and harder to kill.