Holy damage

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

Caused/Changed by Skills
Some Smiting Prayers Monk skills such as Bane Signet cause holy damage. Judge's Insight changes all attack's damage into holy damage.

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.