Internal Damage is a special sort of Armor-Negating Damage that hits a poor fellow from the inside out. It is extra tough to prevent; Elemental Resistances that line up can reduce the damage, but that's about it. Mossbody and Mistform don't reduce the damage, for example.

The following deal "internal" damage:

• Extreme pneumonia caused by Sailors' Death
• Getting your blood boiled by Blood Boil
• Having your heart frozen by Frozen Heart
• Having some of your blood replaced with Poison by Blood Poisoning or Venomous Death
• Getting overloaded by Stream of Life and exploding ("instakill")
• Getting transmuted into dust by Disintegrate ("instakill")

Note that the things Internal Damage bypasses are generally not strong enough to prevent an "instakill" from instantly killing, even with Luck.