Cold Iron

The damage dice is one die lower (d8 becomes d6, etc.) against most creatures for Cold Iron weapons, but against a Fey these weapons are devastatingly effective.   

When a Fey is damaged with a Cold Iron weapon, the weapons damage dice (bludgeoning, piercing, or slashing) are treated as if they rolled the maximum (d8 does 8 damage, etc.) this effects critical damage and sneak attack, but not things like the fire damage from a Flametongue weapon, or a Paladin's smite damage.   

Because of the danger a Cold Iron weapon possess to Fey, they are considered an eminent threat, and a sure sign of hostility among Fey. Bringing a Cold Iron weapon into the home of a Fey, or into one of the Fey Courts is a considered a crime and the law breaker will be treated accordingly.