From GuildWiki

Properties:

• Dark damage is a damage type caused by Necromancer and Ritualist weapons only.
• Dark damage respects armor.
• None of the available weapons or armor upgrades provide specific defenses against it.

Caused by

• Necromancer Staves:
  • Cruel Staff
  • Wicked Staff
  • Accursed Staff
  • Ghial's Staff
• Necromancer Wands:
  • Truncheon
• Ritualist Staves
  • Communing Staff
  • Alem's Remedy
  • Tormented Staff (restoration magic)
• Ritualist Wands:
  • Ritualist Wand

Notes

• Dark damage is not considered elemental damage, and therefore is not affected by skills such as Winter or Ward Against Elements.