D20 Mechanic: Defenses

This section outlines the concept of defenses, a combination of d20 armor class and saving throws, and some mechanics from D&D 4e. Most of the concepts herein are taken from those two systems.

Overview

Creatures have four sets of defenses, each with a number of child values: these are armor class, fortitude, reflex, and will. Armor Class (AC) is further subdivided into its normal value, touch, and flat-footed; of these, some can be conflated, such as an effect being against your flat-footed touch AC.

The remaining three are divided into your defense and your saving throw.

Armor Class

Your Armor Class (AC) represents how hard it is for opponents to land a solid, damaging blow on you. It’s the attack roll result that an opponent needs to achieve to hit you. Your AC is equal to the following:

10 + armor bonus + shield bonus + Dexterity modifier + size modifier

Note that armor limits your Dexterity bonus, so if you’re wearing armor, you might not be able to apply your whole Dexterity bonus to your AC.

Enhancement Bonuses: Wearing Force-aligned armor or wielding a Force-aligned shield increase your AC, granting you an enhancement bonus to your AC equal to your attunement bonus. You can benefit only from a single suit of armor and a single shield.

Deflection Bonus: Some effects can grant you a deflection bonus to AC, as can Force-aligned accessories, equal to your attunement bonus. You only gain this benefit from one accessory.

Natural Armor: Some effects grant you a natural armor bonus to AC. Only the highest bonus applies.

Dodge Bonus: Some effects can grant you a dodge bonus to your AC. Unlike other bonuses, dodge bonuses stack.

Other Bonuses: Several other types of bonus, such as insight bonuses, exist, and can be granted to your AC. Only the highest bonus of a given type is applied at a time.

Touch and Flat-Footed

When you are the target of a touch attack, your AC doesn’t include any armor bonus, shield bonus, or natural armor bonus. All other modifiers, such as your size modifier, Dexterity modifier, and deflection bonus (if any) apply normally.

When you are flat-footed, you lose your Dexterity modifier and all dodge bonuses to AC.

If you are subject to a touch attack while flat-footed, only deflection bonuses to your AC apply.

Other Defenses

Generally, when you are subject to an unusual or magical attack, you get a saving throw to avoid or reduce the effect. Like an attack roll, a saving throw is a d20 roll plus a bonus based on your class, level, and an ability score. Your saving throw modifier is:

Base save bonus + ability modifier

Base Save Bonus: A saving throw modifier derived from character class and level. Base save bonuses increase at different rates for different character classes. Base save bonuses gained from different classes, such as when a character is a multiclass character, stack.

In addition to having a saving throw, you also have a defense value for each of these categories. That is calculated as follows:

10 + base save bonus + ability modifier

Some effects will call for a saving throw, while others will be against your defense of a given type. You cannot choose which of these values to use; a given effect will indicate whether it allows for a saving throw, or is against a given defense.

In addition, some armors can grant a bonus to your defenses. These bonuses are applied to the indicated defenses both as saving throw and defense.

Types

Fortitude 

These saves measure your ability to stand up to physical punishment or attacks against your vitality and health. Apply your Constitution modifier to your Fortitude saving throws.

Reflex 

These saves test your ability to dodge area attacks. Apply your Dexterity modifier to your Reflex saving throws.

Will 

These saves reflect your resistance to mental influence as well as many magical effects. Apply your Wisdom modifier to your Will saving throws.

Defense Effects

There are a number of abilities that impact how your defenses function, which are outlined here.

Evasion

Evasion represents an uncanny ability to quickly remove yourself from danger. It impacts your Reflex.

Evasion: If you make a successful Reflex saving throw against an attack that normally deals half damage on a successful save, you instead takes no damage; if the effect has a partial effect on a successful saving throw, you instead suffer no effect. Evasion can be used only if the you are wearing light armor or no armor, and are not helpless or prone. You gain a +2 bonus to your Reflex Defense.

Improved Evasion: As per Evasion, but in addition, when you fail a Reflex saving throw against an attack or effect, you are instead treated as though having succeeded (though without the benefits of Evasion). Your bonus to your Reflex Defense increases to +5.

Perfect Evasion: You ignore all detrimental effects that call for a Reflex save. Your bonus to your Reflex Defense increases to +10.

Grit

Grit represents a profound ability to withstand physical trauma. It impacts your Fortitude.

Grit: If you make a successful Fortitude saving throw against an attack that normally deals half damage on a successful save, you instead takes no damage; if the effect has a partial effect on a successful saving throw, you instead suffer no effect. Grit can be used only if the you are wearing at least light armor, and are not unconscious. You gain a +2 bonus to your Fortitude Defense.

Improved Grit: As per Grit, but in addition, when you fail a Fortitude saving throw against an attack or effect, you are instead treated as though having succeeded (though without the benefits of Grit). Your bonus to your Fortitude Defense increases to +5.

Perfect Grit: You ignore all detrimental effects that call for a Fortitude save. Your bonus to your Fortitude Defense increases to +10.

Mettle

Mettle represents your mental composure and ability to maintain yourself. It impacts your Will.

Mettle: If you make a successful Will saving throw against an attack that normally deals half damage on a successful save, you instead takes no damage; if the effect has a partial effect on a successful saving throw, you instead suffer no effect. Mettle can be used only if the you are not dazed, stunned, confused, controlled or dominated. You gain a +2 bonus to your Will Defense.

Improved Mettle: As per Mettle, but in addition, when you fail a Will saving throw against an attack or effect, you are instead treated as though having succeeded (though without the benefits of Mettle). Your bonus to your Will Defense increases to +5.

Perfect Mettle: You ignore all detrimental effects that call for a Will save. Your bonus to your Will Defense increases to +10.