Return of the Hydra
Cold iron, mined deep underground, is forged at a lower temperature to preserve its delicate properties. Weapons made of cold iron are known for their effectiveness against Demons and other extraplanar creatures. Cold iron is also anathema to arcane spellcasters and creatures wearing lots of magical stuff. As a result, Erymian wizardslayers are very fond of it, for cold iron is known to even the scales when fighting these ‘magic abusers’.
A cold iron weapon negates x points of damage reduction (as long as the damage reduction is of a type that can be overcome by a non-epic type of weapon), where x is the wielder’s base attack bonus. This only functions as long as its wielder is carrying no magical items or artifacts of any kind.
If you want to use a cold iron weapon, or even carry it around, you cannot have more than 4 magical items in your possession. You cannot have even a single extraplanar artifact. These weapons cannot be used by demons, elementals or outsiders of any other type, and neither by aberrations or magical beasts. (in case this happens, make a will save DC 15 or take 4d6 force damage. The item falls on the ground and you cannot use it again for 24 hours.)
A cleric or other character with the ‘Turn Undead’ or ‘Rebuke Undead’ feature, can spend one use of turn/rebuke undead as a swift action, making the weapon glow brightly for 1 round. During that round, every successful hit against an demon or other outsider, elemental, magical beast or aberration (or against a creature with more than 4 magical items) deals double weapon damage and a morale bonus equal to your charisma modifier on damage.
A normal cold iron weapon costs as much as a normal masterwork one:
Mstw tiny, small and/or simple weapon = base price + 100
Mstw large and/or exotic weapon = base price + 300
All others = base price + 200
These weapons can never be magical, and, due to the difficulties of making such weapons, a masterwork cold iron weapon costs 4 times as much as a normal one.
Mstw tiny, small and/or simple weapon = 4x (base price + 100)
Mstw large and/or exotic weapon = 4x (base price + 300)
All others = 4x (base price + 200)