Constitution

Constitution represents the fundamental physical health, stamina, and vital force of a creature. It is the primary factor in determining a creature’s durability against physical trauma, their capacity to recover from injury, and their internal fortitude against biological or magical afflictions. Beyond simple survival, Constitution dictates a creature’s ability to maintain focus under the strain of combat, withstand the rigors of travel, and push their body’s biological limits without succumbing to exhaustion or systemic failure.

Vitality and Recovery

Hit Points (HP) represent a complex blend of physical durability, mental resolve, and the sheer will to remain standing. They are not merely a measure of "meat" or blood lost; instead, they function as a pool of vitality that allows a creature to turn a lethal strike into a glancing blow or push through the shock of an injury. As long as a creature possesses at least 1 Hit Point, they maintain full combat effectiveness. At 0 Hit Points, the body's internal systems have reached a breaking point, and the creature begins to succumb to the dying process.

Hit Point Maximum

A creature's Hit Point maximum is determined by their class’s Hit Die and their Constitution modifier. At 1st level, the maximum is equal to the highest possible value of the Hit Die plus the Constitution modifier. For every level gained thereafter, the maximum increases by the result of a Hit Die roll (or the fixed average) plus the Constitution modifier.

If a creature’s Constitution modifier changes—whether through permanent ability score improvements, magical items, or debilitating effects—their Hit Point maximum is immediately recalculated as if they had the new modifier starting from 1st level.

Short Rest Recovery

Constitution serves as the primary engine for recovery during short breaks in activity. During a Short Rest, a creature can expend one or more of their Hit Dice to regain lost HP. For every Hit Die spent, the creature rolls the die and adds their Constitution modifier to the result. This represents the body’s innate ability to mend minor wounds, manage systemic shock, and stabilize after periods of intense exertion.

Endurance and Resistance

Unlike standard skills, Endurance and Resistance function as passive proficiencies rather than active, roll-based skill checks. They represent a character’s innate physical reinforcement and are utilized whenever the body is forced to withstand specific hazards or internal strain.

Proficiency Stacking

These skills utilize a unique mechanic called Proficiency Stacking. If a character is already proficient in a Saving Throw (such as Constitution or Dexterity) and the hazard also falls under the domain of Endurance or Resistance, the character adds their Proficiency Bonus for both. For example, a character proficient in Constitution saving throws and the Resistance skill would add their proficiency bonus twice to a roll against poison. If they are only proficient in the skill and not the specific saving throw, they still add the proficiency bonus once to the attempt.

Endurance

Endurance governs the body’s capacity to absorb physical shock and maintain high-intensity output over long durations. This proficiency is applied to saving throws or checks made to mitigate force damage from high-altitude drops and concussive explosions, including the Dexterity saving throw required to reduce damage from a fall. It also reinforces the Constitution check made during a Block maneuver, the maintenance of prolonged activities like sprinting and swimming, and the resistance of exhaustion caused by extreme physical strain or travel.

Resistance

Resistance represents the body’s internal defenses against biological, chemical, and magical interference. This proficiency is applied to saving throws against debilitating internal conditions like being paralyzed, petrified, or stunned. It also aids in surviving elemental damage types such as cold, fire, acid, lightning, and poison. Because this skill focuses on the nature of the damage rather than the movement used to avoid it, it can stack with various saving throws; a creature may add their Resistance proficiency to a Dexterity saving throw to reduce damage from a fireball or a Constitution saving throw to resist a lethal toxin.

Blocking

Blocking allows a character to actively mitigate damage from incoming physical threats by using their body as a literal shield. This maneuver is specifically designed to counter direct attack rolls and the concussive force of explosions, though it cannot be used against standard Area of Effect (AoE) magical effects or environmental hazards that bypass physical bracing.

When a creature is hit by an applicable attack, they may use a reaction to perform a block. A unique aspect of this mechanic is that it does not consume the creature's reaction for the round, allowing them to remain defensive against multiple attackers or still utilize a standard reaction for other abilities later in the turn. However if a creature has already used their reaction this round they can not Block.

To determine the success of the maneuver, the creature makes a Constitution check contested by the attacker’s total attack roll. If this is a feat of physical absorption, characters proficient in the Endurance skill add their proficiency bonus to this check. Same goes for Resistance for incoming elemental attacks. If the Constitution check exceeds the attacker's roll, the creature successfully braces for the impact and halves the incoming damage from that specific source.

Trauma and Injuries

Constitution determines a creature's ability to withstand massive physical trauma and prevents lingering damage. A Minor Injury is risked when taking damage equal to or greater than half of remaining Hit Points (minimum 5 damage), while a Major Injury is risked when damage meets or exceeds half of the total Hit Point maximum. If a single hit qualifies for both, only the Major Injury is applied. Non-lethal damage does not inflict injuries.

To avoid an injury, a creature must succeed on a Constitution saving throw with a DC of 15 or half the damage taken, whichever is higher. Endurance applies to physical or force damage, while Resistance applies to elemental, biological, or magical damage. If a creature is proficient in both the saving throw and the applicable skill, they add their proficiency bonus twice. Unlike standard HP loss, injuries are semi-permanent and require professional medical attention or restorative spells to mend.

Physiological Limits

Constitution governs the body’s basic biological timers and survival thresholds.

A creature can hold its breath for a number of minutes equal to 1 + Constitution modifier, with a minimum of 30 seconds. Once out of breath, the creature survives for a number of rounds equal to its Constitution modifier (minimum 1 round) before dropping to 0 HP and dying.

For long-term survival, a creature can go without food for a number of days equal to 3 + Constitution modifier and requires standard water intake to avoid the immediate onset of exhaustion.

When a creature that is not space bound is left in the vacuum of space the creature survives for a number of rounds equal to its Constitution modifier (minimum 2 rounds) before dropping to 0 HP and dying.

Physical Exertion and Travel

Constitution dictates how far a body can be pushed during high-stress activities and long-distance travel.

In a chase, a creature can use the Dash action a number of times equal to 3 + Constitution modifier before needing to succeed on a DC 10 Constitution check for each additional Dash to avoid exhaustion. If a creature refuses to stop dashing after failing a Constitution check they will take a level of exhaustion, same goes for sprinting.

Traveling more than 8 hours in a day constitutes a Forced March, requiring a Constitution saving throw at the end of each extra hour with a DC of 10 + 1 for each hour past the limit.

Going 24 hours without a Long Rest requires a DC 10 Constitution saving throw to avoid exhaustion, with the DC increasing by 5 for every subsequent 24-hour period of sleep deprivation.

Genetic Corruption

Genetic Corruption represents permanent structural damage to a creature's biology caused by external mutagens, radiation, or the voluntary consumption of Serums.

Corruption Capacity

A creature can safely sustain a maximum number of Genetic Corruption points equal to their Constitution modifier (minimum of 1). If a creature's total corruption exceeds this capacity, they suffer severe systemic strain. While over capacity, the creature gains 1 level of exhaustion at the end of every Long Rest, and they are completely unable to remove any levels of exhaustion.

Serums

Serums offer a voluntary path to gaining Genetic Corruption. In exchange for permanent biological traits, a creature takes on a specific amount of corruption based on the Serum's grade, typically ranging from 1 to 10 points.

Purging Corruption

Genetic Corruption cannot be cured through standard rest. It can only be reduced or removed through three distinct methods:

1. Healing Items: Specific medical treatments and items can purge corruption caused purely by environmental hazards like radiation or blights.
2. Genetic Therapy: Permanent corruption tied to acquired Serum traits can be slowly reduced over time through regular treatment by a professional geneticist.
3. Trait Ablation: A rare and highly expensive surgical procedure can completely remove a Serum trait from the body, fully refunding all Genetic Corruption points associated with it.