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d20 Modern System Reference Document


Darkness and Light

It’s a rare mission that doesn’t end up in the dark somewhere, and heroes need a way to see. See Table: Light Sources for the radius that a light source illuminates and how long it lasts.

Table: Light Sources

Item Light Duration
Candle 5 feet 12 hours
Torch 20 2 hours
Halogen lantern 40 feet 24 hours
Flashlight 20 feet* 6 hours
* Creates a beam 30 feet long and 5 feet high.

Heat and Cold

Heat and cold deal damage that cannot be recovered until the character counteracts or escapes the inclement temperature. As soon as the character suffers any damage from heat or cold, he or she is considered fatigued.

A character not properly equipped to counteract the heat or cold must attempt a Fortitude saving throw each hour (DC 15, +1 for each previous check). Failure means that the character loses 1d4 hit points. Heavy clothing or armor provides a –4 penalty on saves against heat but grants a +4 equipment bonus on saves against cold. A character who succeeds at a Survival check (DC 15) gains a +4 competence bonus on the save (see the Survival skill).

Searing heat or bitter cold (desert or arctic conditions) forces a character to make a Fortitude save every 10 minutes. Failure means that the character loses 1d6 hit points. Appropriate clothing and successful use of the Survival skill can modify the save, as noted above.

Catching on Fire

Heroes exposed to open flames might find their clothes, hair, or equipment on fire. Heroes at risk of catching fire are allowed a Reflex saving throw (DC 15) to avoid this fate. If a hero’s clothes or hair catch fire, he or she takes 1d6 points of damage immediately. In each subsequent round, the burning hero must make another Reflex saving throw. Failure means he or she takes another 1d6 points of damage that round. Success means that the fire has gone out. (That is, once the character succeeds at the saving throw, he or she is no longer on fire.)

A hero on fire may automatically extinguish the flames by jumping into enough water to douse him or herself. If no body of water is at hand, rolling on the ground or smothering the fire with blankets or the like permits the hero another save with a +4 bonus.

Starvation and Thirst

Sometimes heroes might find themselves without food and water. In normal climates, heroes need at least 1/2 gallon of fluids and about 1/4 pound of decent food per day to avoid the threat of starvation. In very hot climates, heroes need two or three times as much water to avoid dehydration.

A character can go without water for one day plus a number of hours equal to his or her Constitution score. After this, the character must make a Constitution check each hour (DC 10, +1 for each previous check) or take 1d6 points of damage.

A character can go without food for three days, in growing discomfort. After this, the character must make a Constitution check each day (DC 10, +1 for each previous check) or sustain 1d6 points of damage.

Damage from thirst or starvation cannot be recovered until the hero gets water or food, as needed. Even magical or psionic effects that restore hit points cannot heal this damage.

Suffocation and Drowning

A character in an airless environment (underwater, vacuum) can hold his or her breath for a number of rounds equal to his or her Constitution score. After this period of time, the character must make a Constitution check (DC 10) every round to continue holding his or her breath. Each round, the DC of the Constitution check increases by 1.

When the character fails one of these Constitution checks, he or she begins to suffocate or drown. In the next round, the character falls unconscious with 0 hit points. In the following round, the character drops to –1 hit points and is dying. In the third round after failing the check, the character dies of suffocation or drowning.


Characters breathing heavy smoke or similar toxic gases must make a Constitution check (DC 10, +1 for each previous check) each round or spend that round choking and coughing. Characters who choke for 2 consecutive rounds take 1d6 points of damage.

Smoke also obscures vision, giving one-half concealment (20% miss chance) to characters within it.


When a character is strangled by an instrument or an attacker, use the rules below.

A character can strangle or choke a target of the same size category or one size category larger or smaller. The strangling attempt incurs an attack of opportunity.

To begin the choke, the attacker must succeed at an opposed grapple check. If the grapple succeeds, the attacker can choose to deal normal unarmed damage as well as choke the target. The target can hold his of her breath for a number of rounds equal to his or her Constitution score. After this period of time, the target must make a Constitution check (DC 10, +1 for each previous check) every round to continue holding his or her breath. The target begins to suffocate on a failed check (see Suffocation and Drowning).

If at any time the target breaks free or slips free of the grapple, the stranglehold is broken (although any damage that was dealt remains). Note that a grappled target who is not pinned can use his or her attack action to strangle his or her attacker.


A character takes 1d6 points of damage for every 10 feet of a fall, to a maximum of 20d6 points. If the character succeeds on a Reflex saving throw (DC 10, +1 for each 10 feet fallen), this damage is halved. If the saving throw fails, full damage is applied.

A character can make a Tumble check (DC 15) to treat a fall as if it were 10 feet shorter when determining the damage and Reflex saving throw DC required by the fall.

Falling Objects

Objects that fall upon characters (or creatures or vehicles) deal damage based on their size and the distance fallen, as noted on Table: Damage from Falling Objects.

Objects deal the initial damage given in Table: Damage from Falling Objects if they fall 10 feet or less. An object deals an additional 1d6 points of damage for every 10-foot increment it falls beyond the first (to a maximum of 20d6 points of damage). Objects of Fine size are too small to deal damage, regardless of the distance fallen.

A successful Reflex save indicates that the target takes half damage. The size of the falling object determines the save DC.

If the save fails by 10 or more, and the object is at least three size categories larger than the character, the character is pinned under the fallen object. A pinned character cannot move but is not helpless. The character can make a Strength check to lift the object off him or herself or an Escape Artist check (DC 20) to get out from underneath. The GM can modify the DCs for these checks based on the circumstances.

Table: Damage from Falling Objects

Object Size Examples Initial Damage Reflex Save DC Strength Check DC
Fine Penny 0 n/a n/a
Diminutive Paperweight 1 0 n/a
Tiny Wrench 1d3 5 n/a
Small Vase 1d4 10 5
Medium-size Briefcase 1d6 15 10
Large Garbage Can 2d6 20 20
Huge Oil Barrel 4d6 25 30
Gargantuan Piano 8d6 30 40
Colossal Vehicle 10d6 35 50


When a character takes damage from an attack with a poisoned weapon, touches an item smeared with contact poison, consumes a poisonous substance, inhales a poisonous gas, or is otherwise poisoned, the character must make a Fortitude saving throw. If the character fails, he or she takes the poison’s initial damage (usually ability damage). Even if the character succeeds, he or she typically faces secondary damage 1 minute later. This secondary damage also requires a Fortitude saving throw to avoid.

Poisons are detailed in the Craft(chemical) skill description.

Poisonous liquids are usually administered through injection or by application to a weapon. Poisonous gases must be inhaled to be effective. Poisonous solids are usually ingested with food or drink.

Perils of Using Poison: A character has a 5% chance (roll of 1 on 1d20) to expose him or herself to a poison whenever the character applies it to a weapon or otherwise readies it for use. Additionally, a character who rolls a 1 on an attack roll with a poisoned weapon must succeed at a Reflex saving throw (DC 15) or accidentally poison him or herself with the weapon.

Poison Immunity: Creatures with natural poison attacks are immune to their own poison. Nonliving creatures and creatures without metabolisms are immune to poison. Oozes and certain kinds of creatures are immune to poison, as detailed in their descriptions, though it is conceivable that a special poison could be synthesized specifically to harm them.


When a character is exposed to a treatable disease, the character must make an immediate Fortitude saving throw. The victim must make this roll when he or she comes into contact with an infectious carrier, touches an item smeared with diseased matter, consumes food or drink tainted with a disease, or suffers damage from a contaminated attack. If the character succeeds, the disease has no effect on him or her—the character’s immune system fights off the infection. If the character fails the save, he or she takes damage after an incubation period; once per day thereafter, the character must succeed at a Fortitude saving throw to avoid secondary damage. Two successful saving throws in a row indicate that the character has fought off the disease and recovers, taking no more damage.

The characteristics of some treatable diseases are summarized on Table: Diseases.

Type: The disease’s method of delivery—ingested, inhaled, or via an injury—and the DC needed to save. Some injury diseases can be transmitted by a wound as small as an insect bite. Most diseases that are inhaled can also be ingested (and vice versa).

Incubation Period: The amount of time before initial damage takes effect (if the victim fails his or her Fortitude save).

Initial Damage: The damage the victim takes after the incubation period.

Secondary Damage: The amount of damage the hero takes one day after taking initial damage, if he or she fails a second saving throw. This damage is taken each day the saving throw fails.

Table: Diseases

Disease Type Incubation Period Initial Damage Secondary Damage
Anthrax Inhaled/Injury DC 16 1d2 days 1 Con 1d4 Con*
Small pox Inhaled/Contact DC 15 2d4 days 1 Str and 1 Con 1d2 Str and 1d2 Con
Pneumonia Inhaled DC 12 1d4 days 1 Str 1d3 Str and 1d3 Con
Hantavirus Injury DC 14 1 day 1d2 Str 1d2 Str* and 1d2 Con*
Necrotizing faciitis Contact DC 13 1d6 days 1 Con 1d3 Con*
West Nile virus Inury DC 12 1d4 days 1 Dex and 1 Con 1d2 Dex and 1d2 Con*
Salmonellosis Ingested DC 13 1 day 1 Str and 1 Dex 1 Str and 1d3 Dex
* If damage is sustained, make a second saving throw to avoid 1 point being permanently drained (instead of damaged).


Corrosive acids deal damage each round of exposure. The amount of damage varies depending on the acid’s strength, as noted on Table: Acid Damage.

Table: Acid Damage

Acid Strength Splash Attack* Total Immersion*
Mild 1d6 1d10
Potent 2d6 2d10
Concentrated 3d6 3d10
* Damage per round of exposure.

Acid damage from an attack reduces hit points. A character fully immersed in acid takes potentially more damage per round of exposure than a character splashed with acid.

The fumes from most acids are inhaled poisons. Those who come within 5 feet of a large body of acid must make a Fortitude save (DC 15) or take 1 point of temporary Constitution damage. A second save must succeed 1 minute later to avoid taking another 1d4 points of Constitution damage.


Electrical hazards come in many forms, including stun guns, downed power lines, and electric security fences. Table: Electricity Damage gives damage values for various electrical hazards based on relative voltage. A character can make a Fortitude saving throw to reduce the damage by half. If that character is not grounded or is otherwise insulated from the current, a successful save indicates that no damage is suffered.

Table: Electricity Damage

Type Examples Damage Fort DC
Jolt Car battery, stun gun 1d3 10
Low voltage Fuse box, electrical socket 2d6 15
Medium voltage Industrial transformer, electric fence 4d6 15
High voltage Power line, electric chair, lightning 8d6 20