Holy damage

Caused by Weapons
Monk rods, specifically Smiting Rods with a Smiting Prayers skill requirement inflict holy damage.

Caused/Changed by Skills

 * Scourge Healing
 * Balthazar's Aura
 * Symbol of Wrath
 * Bane Signet
 * Signet of Judgement
 * Holy Strike
 * Smite
 * Banish
 * Judge's Insight

Armor modifications on

 * Some creatures (undead, minion..) have less armor against holy damage.
 * The Necromancer's Tormentor's Armor and Necrotic Armor have the weakness: "reduces protection from holy damage", whatever that means exactly.