Damage is mostly used to reduce the hit points of a unit in an attempt to kill them. It is applied as a result of successful attacks and combat spells. Damage always has a damage type. It has a number of possible types and additional modifiers, all of which are indicated on the details window for each weapon or spell. Some of the more common damage tags can be found here. A wide range of abilities and spell effects may alter the damage calculation in some way both before and after this protection roll.
A basic melee attack with no additional damaging effects or protective effects is a roll of the attacker's weapon damage vs the defender's protection:
Protection roll: Defender's Protection value
+ Shield Protection if it is a shield hit
The Protection value used is the target's listed Protection value, unless it is a head hit. In case of a headshot, the target's helmet Protection value is used instead of the main Protection value.
If an attack has been Parried with a shield, the shield Protection is still added. The blow is assumed to have glanced off the shield and hit the target in the head. If the damage roll is greater than the protection roll, the difference is subtracted from the target's hit points. If the protection roll is equal to or greater than the damage roll, there is no effect.
Any time damage is rolled, a check is made to see whether a critical hit was scored. Units that suffer from heavy fatigue or are immobilized are easier to crit. Every time an attack hits, the defender's DRN roll is checked against its fatigue or if it is immobilization. If successful the target's Protection is decreased by 25% multiplicatively. So a unit hit with an armor piercing critical hit will have an effective (0.5 * 0,75 = ) 37,5% Protection.
Defender roll: Defender's Protection DRN roll
- 1 if fatigue $\ge$ 50
-1d6 exploding with cursed luck (50% chance)
Crit value: If defender fatigue $\ge$ 100
or defender is immobilized ⇒ Crit value = 4
Else ⇒ Crit value = 2
If Defender roll $\le$ Crit value ⇒ Critical Hit!
If an attack was a critical hit the defender's Protection is decreased by 25%