Throughout the Thunder Rift campaign, and Dungeons & Dragons in general, there are various creatures and classes that have Damage Reductions (DR). A Damage Reduction is how much an normal attack's damage is reduced before being applied to the target creature or object. For example, a 4th-level barbarian has a DR of 1, so all damage he takes in a round from each normal attack is reduced by 1 point. Damage Reductions are displayed by the abbreviation DR, followed by the amount of damage it is reduced by, followed finally by whatever can overcome that Damage Reduction. So the 4th-level barbarian's Damage Reduction of 4 would be displayed as DR 1/- (the "-" indicating there is no special weakness for this DR).
Some DR's, such as those for lycanthropes or outsiders, have weaknesses to certain types of materials. Wererats, for example, a lycanthrope, has a Damage Reduction of 15, but has a weakness to silver weapons. This Damage Reduction would be displayed as DR 15/silver, meaning that against all singular attacks the wererat's Damage Reduction reduces damage by 15 points, unless they are being attacked by a silver weapon, in which case the silver weapon bypasses the Damage Reduction and does full damage.
In this section we are going to look at Damage Reductions, how they are affected by rare materials, streamline some of the Damage Reduction rules (which truthfully can be a little muddled at time) and the affect of magic on them.
SILVER
The first material to discuss is the most basic, silver. Of the materials listed here, though it is precious enough to use as money, silver is the most easily found of all the materials found here. Humans, dwarves, hobbits, elves, almost every race found in Thunder Rift has access to silver, whether they mine it or trade for it. Weapons forged from silver are soft and inherently weaker than their steel counterparts (Hardness of 9 and 15 hit points per inch of thickness). Silver weapons, by their nature, are not inherently special but most weaponsmiths make the time and effort to forge masterwork weapons when they are working with silver. Silver is said to be a "purifying metal," something that can affect cursed or tainted creatures that walk the Known World.
Silver weapons cost x5 the weapon's base price (before the additional cost for masterwork weapons).
Silver and DR: Some creatures, most notably lycanthropes, have a specific weakness to silver. As mentioned previously, anything with a Damage Reduction that has silver mentioned in its description is weak to silver. In addition to anything with silver noted as a weakness, silver can affect the undead as well. Any creature with the lycanthrope template or undead type are affected by silver, even by those that are listed as being weak only to magical weapons. Silver weapons can affect undead (bypass DR) that are weak to magical weapons up to and including +3. What this means is that even normal, non-masterwork, silver weapons can bypass the DR of undead creatures that are weak to only +1 to +3 magic weapons.
COLD IRON
Cold Iron is one of the second most common materials found. Cold iron is the name given to an iron ore than can rarely be found in the ground along side normal veins of iron. Called cold iron due to the need to forge with metal into a weapon without firing it. If heat is applied to cold iron during the forging process it will ruin the weapon and turn the ore into regular iron. The appearance of cold iron has distinctive patterns of banding and mottling that resembles flowing water (this is similar in appearance to real world Damascus steel and is used to differentiate the appearance of cold iron from standard weapons). Where silver is said to have properties that purify and affect the cursed or impure of the world, cold iron is said to have properties that "calm the chaotic." This does not reference alignment, but rather things that are native or natural to the Known World. The most notable creatures to be affected by cold iron are therianthropes (creatures like the wolfwere and jacklewere). Like silver, there is nothing inherently remarkable about weapons forged from cold iron, but many weaponsmiths will take the effort to create masterwork weapons from cold iron.
Cold iron has a hardness of 10 and 30 hit points per inch of thickness.
Cold Iron is rare, but it is no more difficult to work with than standard iron. Weapons made of cold iron are x3 the weapon's base price.
Cold Iron and DR: Just like with silver, there are some creatures that are affected only by cold iron weapons, like the aforementioned therianthropes. Other creatures susceptible to the bite of cold iron are outsiders (whether good or evil), as well as the fey and those native to the Other World, Tir na Nog. Like silver, cold iron can affect certain creatures normally only affected by magic weapons. Weapons forged of cold iron, even mundane weapons, can cause damage to fey and outsiders that are only damaged by weapons that need a +1 to +3 enchantment bonus, much the same way silver can affect the undead.
DARKWOOD
Darkwood is a strong wood with an exceedingly dark brown, almost black, coloration. Darkwood trees do not grow in groves, instead they are usually singular trees found in the middle of forests that are, in one form or another, enchanted. Within Thunder Rift the most obvious place to locate darkwood would be the Gauntlin Forest. Darkwood is as strong as normal wood but very light (weapons and items made of darkwood weigh half as much as normal, rounded down). As with silver and cold iron, darkwood has no special properties, however those who fashion weapons from them, such as spears, arrows, and bows, often fashion masterwork items from the material. To determine the cost of a masterwork weapon made of darkwood, use the original weight of the weapon and add +10 gp per pound.
Darkwood has a hardness of 5 and 10 hit points per inch of thickness.
Darkwood and DR: Darkwood weapons have no special affect on Damage Reductions. Darkwood weapons will inflict damage upon vampires no matter the undead's DR or vulnerabilities. Many a stake in a vampire's chest are made of darkwood.
MITHRIL
Of all the materials listed, mithril is without a doubt the most rare of all materials here. Found only in the dwarven realm of Dwarland. The properties of mithril are such that it can affect creatures like lycanthropes and outsiders the same way that silver does, including being able to damage them through DR's that require a +1 to +3 enchantment. Mithril is lightweight, strong, and requires time to work into weapons and armor. Weapons fashioned of mithril are treated as masterwork with regard to creation time, but this does not affect the enhancement bonus. To gain a +1 enhancement bonus due to masterwork construction the weaponsmith must double the cost and time.
Mithril has a hardness of 15 and 30 hit points per inch of thickness. Weapons made of mithril weigh half as much as their normal counterparts.
Mithril and DR: As previously stated, mithril can has the same affects against creatures and DR as silver does.
MAGIC WEAPONS
Magic weapons are by far the easiest and most common way to bypass most Damage Reductions. The construction of magical weapons and the various different types of magical weapons is listed in the Dungeon Master's Guide (pages 183 - 190), so we won't go deep into that here. We will say, what most people understand, is that the higher the "+" bonus, typically the higher the capabilities of the magic weapon. (Everything listed here for magic weapons applies to psionic weapons as well).
Magic Weapons and DR: How magic and psionic weapons overcome a creature's Damage Reduction is fairly self evident. A creature with a Damage Reduction of DR 5/+1 means that it will negate the first 5 points of damage from an attack unless the attacker is wielding a weapon with a +1 enchantment or better. Likewise a creature with a Damage Reduction of DR 20/+5 will negate the first 20 points of damage from an attack, even if the attacker is wielding a +1 through +4 magical weapon; the attacker would need to be wielding a weapon with a +5 enchantment.
Magic Weapons and DR Vulnerable to Materials: In previous entries it was explained that some creatures, such as lycanthropes and therianthropes are only vulnerable to silver and cold steel respectively (and indeed there is the possibility of encountering other creatures vulnerable to other exotic materials). A cavalier fighting a werewolf and wielding a +1 longsword would be able to damage the werewolf as if he were wielding a silver weapon; magic weapons will always damage a creature normally if they have a material weakness in their Damage Reduction. However, they will not be able to kill them. A character striking a creature like a lycanthrope with a magic weapon can take the opponent to 0 hit points, but cannot kill them. The aforementioned werewolf, being struck by a +1 longsword, and brought to 0 hit points would be staggered (Dungeon Master's Guide page 85) until he can heal. To kill a creature that is vulnerable only to special materials, the killing blow must be dealt with said material. For example, the werewolf brought to 0 hit points by the +1 longsword would have be taken below 0 hit points with a weapon made of silver to kill him.
Silver as Armor
Silver is a weak metal, softer and less resilient than steel or iron. As such a metal, silver makes for poor armor. Armors that are made from silver are mostly ceremonial and never intended for use in battle.
Silver armor typically has 1 less point of Armor Class than armor made of steel or iron, but still has the additional cost. Silver armor that has been enchanted retains the base AC of an armor of it's type.
As with silver weapons, silver armor costs x5 armor type's base price (before the additional cost for masterwork suits of armor).
Cold Iron as Armor
As with cold iron weapons, armor is no more difficult to make than standard iron or steel. Likewise, armor made of cold iron is no more or less protective than its mundane counterpart.
The advantage of cold iron armor over its mundane counterpart is against the touch attacks of outsiders. A character wearing cold iron armor receives a +2 bonus to saving throws against outsider or fey touch attacks, such as a gelugon's cold attack or a succubus's energy drain.
Darkwood as Armor
Wood in general is not a good substance to make armor from, however darkwood does make excellent shields. Armor check penalties for darkwood shields are all reduced by 2.
Mithril as Armor
Though mithril is sought after and prized for weapons, when it is forged into armor is when the true value of mithril is realized. Mithril armor is always treated as one category lighter than normal for purposes of movement and other limitations (for example, whether a barbarian can use her fast movement ability while wearing the armor or not). Heavy armors are treated as medium, and medium armors are treated as light, but light armors are always treated as light. Spell failure chances for armors and shields made from mithril are decreased by 10%, maximum Dexterity bonus is increased by 2, and armor check penalties are decreased by 3.
Cost adjustment for mithril weapons and armor vary:
Light Armor: +1,000 gp
Medium Armor: +4,000 gp
Heavy Armor: +9,000 gp
Shield: +1,000 gp
Other Items: +500 gp/lb
DR and Magic
As a general rule, magic, whether it is arcane or divine, can harm and kill creatures through their Damage Reduction, no matter what it may be. The same can be said of even mundane energy attacks, like fire and electricity.