When a damage is dealt, either by a successful attack, a spell, or other means, a damage roll is made to determine the severity of damage and the results are subtracted from the target’s total hit points. When the opponent’s hit points reach 0, they begin dying.
Some abilities grant you temporary hit points. These extra hit points do not get added to your normal hit point total. Any damage taken gets taken from these points before your actual hit point total is affected. Multiple temporary hit points don't stack.
Damage can be classified into the following types
Acid: Any corrosive liquid that eats away at you.
Bleed: damage from open wounds that occur regularly each round after inflicted. Bleed damage is healed by a DC 15 Medicine check or any magic healing. Bleed damage does not stack with any other active, bleed damage.
Bludgeoning: Blunt-force damage from a physical weapon.
Cold: Ice, frost, freezing of any sort.
Radiant: divine energy that wounds the enemies of a deity
Electricity: Lightning, electricity, shock damage.
Fire: Fire, flames, lava, or heat damage.
Force: Pure magical energy.
Necrotic: Magic that directly attacks the life force of a being.
Piercing: Arrows, thrusts, bites, or other puncturing damage.
Poison: Harmful chemicals, toxins, venom, gas that wound from the inside.
Psychic: Direct attacks on the mind
Slashing: Edge damage from swords, knives, axes, etc.
Sonic: Sound-based damage.
Attacks that deal precision damage take extra care to inflict damage in a place where it hurts the most. The damage dealt by these attacks still falls under one of the standard damage types listed above, but the manner in which the damage is delivered requires precision. The following rules apply to precision damage.
Most creatures take regular damage from each of these types. Some creatures, however, are immune to particular damage types, taking no damage when hit.
Some creatures are resistant to certain types of damage. So, a creature with Fire 10 resistance will negate the first 10 points of fire damage done to it on each attack. If the attack does fewer than 10 points of damage, the creature takes no damage at all. The most common form of resistance is resistance to weapon damage (which includes Piercing, Bludgeoning, and Slashing combined).
Conversely, some creatures are vulnerable to certain damage types. Each successful attack that deals damage from a vulnerable source will deal extra damage. For example, a creature with Piercing 5 will suffer 5 extra hit points of damage every time it is hit by an arrow.
Sometimes creatures are resistant or vulnerable to a damage source instead of a damage type. Sources can be silver weapons, cold iron weapons, magic weapons, or other sources. Any damage done from a weapon with these sources also deals vulnerability damage.
Spell resistance (abbreviated SR) is the extraordinary ability to avoid being affected by spells. Some spells also grant spell resistance. To affect a creature that has spell resistance, a spellcaster must make a spell attack check (d20 + spell attack) at least equal to the creature's spell resistance. The defender's spell resistance is like an Armor Class against magical attacks. If the caster fails the check, the spell doesn't affect the creature. The possessor does not have to do anything special to use spell resistance. The creature does not need to even be aware of the threat for its spell resistance to operate.
A creature can voluntarily lower its spell resistance. Once a creature lowers its resistance, it remains down until the creature's next turn. At the beginning of the creature's next turn, the creature's spell resistance automatically returns unless the creature intentionally keeps it down. A creature's spell resistance never interferes with its own spells, items, or abilities.
Some classes, such as Rogues, have the ability to inflict additional precision-based damage on enemies under certain circumstances. Essentially, if a rogue can catch an opponent when he is unable to defend himself effectively from her attack, she can strike a vital spot for extra damage. This extra damage applies to every attack made against an opponent, not just the first one in a round. Sneak attack damage is not applied for attacks made with the offhand.
The rogue's attack deals extra damage anytime her target is Flat Footed, or when the rogue and one other player both threaten an enemy at melee distance. Should the rogue score a critical hit with a sneak attack, this extra damage is not multiplied.
The following conditions apply to sneak attack damage.
Diseases, poisons, spells, and other abilities can all deal damage directly to your ability scores. This damage does not actually reduce an ability score, but it does apply a penalty to the skills and statistics that are based on that ability. That is, if your intelligence is damaged, you do not actually become less intelligent, but your mind is hazy and unable to perform at your full potential.
Ability damage may affect one score or it may affect all of them. Physical ability damage inflicts points in each of the three physical ability scores (strength, dexterity, constitution). Mental ability damage does so to each of the mental ones (intelligence, wisdom, charisma).
Damaged: Once you take at least 3 points of damage to any given ability score, all rolls except for attacks that depend on that ability modifier are made at disadvantage. This includes ability checks, saves, skills, initiative rolls, and others.
Severely Damaged: An ability is severely damaged once you have taken damage equal to half your ability score. If strength is severely damaged, your melee attacks are now made at disadvantage. Likewise, ranged attacked are made at disadvantage if your dexterity is severely damaged. Spellcasters with severely damaged casting abilities must make concentration checks in order to cast spells. If your Dexterity is severely damaged, then you are considered flat-footed in all combat situations and your movement is reduced to half speed.
Incapacitated: Once your damage is equal to your ability score, you are incapacitated. If Strength, Intelligence, Wisdom, or Charisma are fully damaged, then you are unconscious and unable to function. If Dexterity is fully damaged, you are unable to move but conscious. If your Constitution is fully damaged, you immediately die.
Ability damage does not affect saving throw DCs. Also, Con damage does not decrease a player’s total hit points.
Ability damage can be caused by many sources, both mundane and magical. Regardless of source, before you can recover from ability damage, the cause of the damage must first be removed. If the damage is caused by disease or poison, they must first run their course and be cured. If caused by a magical effect or spell, the spell must be dispelled or be allowed to expire. Likewise, if caused by a curse, the curse must first be removed.
Once the source of damage is removed, your ability scores start to recover naturally. After each long rest, you recover from one point of ability damage for each score affected. After extended rests, you recover two points. Recovery can be doubled if treated by someone who makes a successful Medicine check (see Knowledge).
Injury: Sometimes damage can be caused by more mundane means. A player who falls from a great distance may sprain his foot, or more severely, a player may have his foot amputated, causing Dexterity damage. Before that damage can be healed, the sprain must be treated (using a Medicine check and proper care) or the foot must be regenerated. Once the cause has been repaired, the ability damage can then heal too. Ultimately, the GM will make a judgment call about when major injuries like these happen, and how long they linger before healing begins.
Afflicted Wounds: Afflicted Wounds: Some creatures have the ability to inflict cursed wounds. These afflicted wounds cause ability damage that cannot be healed until the curse is removed from them. Even a restoration spell fails while the curse is in effect. After remove curse or a similar spell is used to lift the curse, then the wounds can be healed normally.
Exposure: Characters who expose themselves to hostile environments for which they aren't prepared may take ability damage as a result. Players who find themselves in the arctic without adequate clothing, for instance, may at the GMs discretion need to regularly make CON saves to prevent physical ability damage. Naturally, this damage doesn't start to heal until the players find a way to protect themselves from the elements.
Some creatures (usually undead) drain the life essence from others when they attack. When successful, these attacks deal normal damage plus they sap your maximum hit points, usually without a saving throw. When you are the victim of life drain, you roll Hit Dice appropriate to your character's class and subtract that number of hit points from your current and max (subtract your Constitution bonus also, if applicable). If you have multiple classes, use the Hit Dice from the class with the most levels.
Life drain does not heal naturally. In order to restore lost hit points from your max, you must use restoration, miracle, or wish spells. Resurrected creatures retain any life drain they accumulated before death.
If life drain takes your current hit points to 0, you fall unconscious and start dying, as usual. If your hit point max reaches 0, you die instantly. Creatures that die from life drain can be resurrected but can only survive 1 round before wasting away and dying again. During this time, a restoration spell can be cast on the creature to bring their hit point max above 0 and prevent re-death.
If you are killed by an undead creature's life drain, you will raise from the dead in 1d4 days as an undead spawn of the same creature type. You can prevent this misfortune if you are resurrected before you raise, but afterward, nothing short of a wish or miracle can restore you to life. Undead spawn usually have 1-2 HD and are under the control of creature that killed them. If spawn's master dies, the undead can use its own life drain abilities to gain Hit Dice (one per HD drained) until it reaches the normal HD max for a creature of its kind.
Hit points represent the general health and willingness to continue living for each creature. Once they make their way to 0, the creature or player is on their way to death.
Each time your hit points drop to 0, you fall unconscious, take 3 constitution damage, and acquire the dying condition. Your turn moves in the initiative order to just before the current creature's turn, giving you a full round before your turn again.
At the beginning of your turn, if you are still dying, you must make a special death save. This is a Con save, except that it suffers no penalties from any Con damage you have. The DC for this saving throw is equal to the save DC of the attacker/effect/trap that knocked you down to 0 hit points. If you fail your death save, you take three points of constitution damage. Once the Con damage you've sustained equals your Constitution score, you die. If you succeed on a death save, you become stable (see below).
Creatures can continue to attack you while you are dying. Each successful attack against a dying creature adds another three points of Constitution damage (critical hits deal double). If you suffer a coup de grace (see above), you take double the ability damage and must save or die. Continual damage (bleed effects, acid, fire) also inflicts an extra three points of con damage at the beginning of your turn, before you make your death save. Any healing cast on a dying creature grants them hit points normally and removes the dying condition.
You become stable if you make a successful death save, if someone casts a stabilize spell on you, or if someone treats you with a successful Medicine check (DC = the DC of the effect that wounded you). Once stable, you no longer have to make death saves every round, though you remain unconscious until you regain hit points.
Any damage you sustain while stable changes your condition to dying and automatically inflicts three points of Constitution damage. One round later, you begin making death saving throws again.
When stable, you may be healed though magical or natural means. Once you have hit points again, you regain consciousness and lose the stable condition
Regardless of whether you are at full health or dying, if you suffer damage from a single effect that equals your max hit points, you automatically die.
Players can take three types of rests: a short rest, a long rest, and an extended rest. All types of rest require light activity, no extended travel, combat, or strenuous activity. Long and extended rests naturally include at least 6 hours of sleep.
Short Rest (1 hour): Players may use a short rest to recover abilities or to treat wounds (see Knowledge: Medicine). If resting players have access to adequate food and water, they gain 1 hp on a short rest. You may only take two short rests in a day. If you take more than two, you fail to gain the benefit from that rest.
Long Rest (8 hours): After a long rest, players recover a number of hit points equal to their character level + CON bonus. Many spell casters get their spells back after a long rest, as well. Players also recover one point of ability damage for each ability affected, provided the cause of the damage is fixed. A long rest must be accompanied by at least 6 hours of sleep to be beneficial. Players who fail to complete a long rest in 24 hours must make a DC 15 Constitution save or gain one level of fatigue. Every 8 hours without a long rest they must continue to make saves, incurring another level of fatigue each time they fail.
Extended Rest (24 hours): Players recover twice the hit points and ability points they usually recover from a long rest (4x the rate if someone provides care using a successful Medicine Check).