PHB:
damage reduction (DR): A special defense that allows a creature
to ignore a set amount of damage from most weapons, unarmed
attacks, or natural weapons, but not from energy attacks, spells,
spell-like abilities, and supernatural abilities. The number in a
creature’s damage reduction is the amount of hit points of damage
the creature ignores. The information after the slash indicates the
type of weapon (such as magic, silver, or evil) that overcomes the
damage reduction. Some damage reduction, such as that of a
barbarian, is not overcome by any type of weapon.
MM:
Damage Reduction (Ex or Su): A creature with this special
quality ignores damage from most weapons and natural attacks.
Wounds heal immediately, or the weapon bounces off harmlessly
(in either case, the opponent knows the attack was ineffective).
The creature takes normal damage from energy attacks (even nonmagical
ones), spells, spell-like abilities, and supernatural abilities.
A certain kind of weapon can sometimes damage the creature normally,
as noted below.
The entry indicates the amount of damage ignored (usually 5 to
15 points) and the type of weapon that negates the ability. For
example, the werewolf’s entry reads “damage reduction 10/silver”:
Each time a foe hits a werewolf with a weapon, the damage dealt
by that attack is reduced by 10 points (to a minimum of 0). However,
a silvered weapon deals full damage.
Some monsters are vulnerable to piercing, bludgeoning, or
slashing damage. For example, skeletons have damage reduction
5/bludgeoning. When hit with slashing or piercing weapons, the
damage dealt by each attack is reduced by 5 points, but bludgeoning
weapons deal full damage.
Some monsters are vulnerable to certain materials, such as
alchemical silver, adamantine, or cold-forged iron. Attacks from
weapons that are not made of the correct material have their
damage reduced, even if the weapon has an enhancement bonus.
Examples: the werewolf ’s damage reduction 10/silver, the iron
golem’s damage reduction 15/adamantine, and the nymph’s damage
reduction 10/cold iron.
Some monsters are vulnerable to magic weapons. Any weapon
with at least a +1 magical enhancement bonus on attack and damage
rolls overcomes the damage reduction of these monsters. Such
creatures’ natural weapons (but not their attacks with weapons)
are treated as magic weapons for the purpose of overcoming
damage reduction. For example, the gargoyle has damage reduction
10/magic and can strike as a magic weapon for the purpose of
overcoming damage reduction.
A few very powerful monsters, such as the solar and the tarrasque,
are vulnerable only to epic weapons; that is, magic weapons
with at least a +6 enhancement bonus. Such creatures’ natural
weapons are also treated as epic weapons for the purpose of overcoming
damage reduction.
Some monsters are vulnerable to chaotic-, evil-, good-, or lawfulaligned
weapons. When a cleric casts align weapon, affected weapons
might gain one or more of these properties, and certain magic
weapons have these properties as well. For example, many tanar’ri
demons and baatezu devils have damage reduction 10/good, while
many celestials have damage reduction 10/evil. A creature with an
alignment subtype (chaotic, evil, good, or lawful) can overcome
this type of damage reduction with its natural weapons and weapons
it wields as if the weapons or natural weapons had an alignment
(or alignments) that match the subtype(s) of the creature. A
bearded devil, for instance, has the evil and lawful subtypes, and
thus can overcome damage reduction as if its weapons and natural
weapons were evil-aligned and lawful-aligned.
When a damage reduction entry has a dash (–) after the slash,
no weapon negates the damage reduction.
A few creatures are harmed by more than one kind of weapon.
The babau demon, for example, has damage reduction 10/cold
iron or good. Either kind of weapon—cold iron or good—overcomes
its damage reduction.
A few other creatures require combinations of different types of
attacks to overcome their damage reduction. The ghaele eladrin has
damage reduction 10/evil and cold iron, meaning that a weapon must
be made of cold-forged iron and be evil-aligned in order to overcome
the ghaele’s damage reduction. A lich has damage reduction 15/bludgeoning
and magic, meaning that only bludgeoning weapons with at
least a +1 enhancement bonus deal full damage to it. A weapon that
falls into one category but not the other is of no help in overcoming
the creature’s damage reduction—a magic sword or a nonmagical
mace is no better at harming a lich than any other weapon.
