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. For details on how your AC score is calculated, see AC calculations.
AC damage reduction
AC reduces the damage of attacks 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 armour in other games, like spell magic.
Guaranteed Damage Reduction
Main Article: GDR
When taking physical damage from a melee attack, 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. GDR only applies to melee attacks.
List of Damage that ignores AC
- Torment and Holy Word.
- Shared damage from Anguish, Injury Mirror, Injury Bond, Pain Bond, or spectral weapons (uses the original target's AC instead).
- Status effects: poison, sticky flame, flay.
- Hex effects: Pain / Necrotise, Mindburst, Anguish, Agony
- Weapon brand damage and monster attack flavours.
- Note that weapon brand damage is often a percentage of AC-reduced damage in the first place.
- Some monster attack flavours require a physical, AC-checking attack to deal damage to apply, though the attack flavour itself is not influenced by AC.
- Specific spells: Freeze, Vampiric Draining, Static Discharge, Ignite Poison, Dispel Undead / Dispel Undead Range, Lee's Rapid Deconstruction (directly on a monster).