Armour class
When something injures you, your Armour class (often abbreviated to AC) reduces the amount of damage you suffer. The higher the value, the better.
The most common way to improve your AC is by wearing armour. Heavier and higher-AC body armour (and body armour alone) comes at the cost of encumbrance, which reduces EV, spell success rates, and stealth. Other pieces of gear, especially artefacts, may also increase your AC, and magical spells may temporarily boost it.
AC damage reduction
In general AC reduces the damage of attacks susceptible to AC by an absolute value between 0 and your current AC value. an AC of 10 will reduce damage by a random number between 0 and 10. This applies to damage that would traditionally ignore defense in other games, such as most spell magic and elemental attacks. However, this is not always precisely true. See the AC calculations page for more details.
Guaranteed Damage Reduction
Main Article: GDR
When taking physical damage in melee, guaranteed damage reduction will improve AC rolls. GDR% is equal to 16*(AC)^1/4. If GDR is applied, the minimum possible AC roll will equal (damage*GDR%) or (AC/2), whichever is lower.
List of Damage that ignores AC
- Torment and Holy Word.
- Smite.
- Damnation.
- Status Effects: poison, sticky flame, flay.
- Weapon brand damage: includes attack flavours.
- Note that weapon brand damage is a percentage of AC-reduced damage in the first place.
- Some attack flavours require a physical AC attack to deal damage to apply, though are not influenced by AC themselves.
- Specific spells: Freeze, Vampiric Draining, Static Discharge, Frozen Ramparts, Ozocubu's Refrigeration.
- The chilling blast breath attack of a white draconian.
- Hex effects: Pain, Anguish
