Shadow damage

caused by Weapons
Shadow damage is caused by necromancer wands and staves

caused by Skills
Shadow Damage caused by necromancer Skills:
 * Dark Pact
 * Deathly Chill (if foes Hp is above 50%)
 * Spiteful Spirit (if foe attacks or uses a skill)
 * Mark of Pain (if foe takes physical damage)
 * Dark Aura (if foe sacrifices Health)
 * Descerate Enchantments (extra damage per enchantment on foe)
 * Touch of Agony

Armor against Shadow Damage
There is no special armor against Shadow Damage, because its neither physical damage nor elemental damage. That makes Shadow damage slightly more powerfull, especially against Rangers and Warriors.