Source: Complete Mage
Your fey heritage guards you against all weapons except those crafted from the dreaded cold iron.
Prerequisite: Non-lawful alignment, Fey Heritage.
Benefit: You gain damage reduction (overcome by cold iron) equal to 1 + the number of feats you have that list Fey Heritage as a prerequisite (including such feats that you take after gaining this one). For example, if you have Fey Skin and Fey Presence, you would have damage reduction 3/cold iron.
This value stacks with any similar damage reduction you might have from your type, subtype, race, or class, but not from other sources, such as spells or magic items.