Ysgardian Heartwire

This incredibly fine, flexible metal is found only in the deep mines of Nidavellir in the Heroic Domains of Ysgard. It is not suitable for making an entire suit of armor, but small sections of heartwire mail can be incorporated into chain shirts, chainmail, or heavy armor to reinforce vital areas. This reinforcement has the effect of increasing the wearer’s AC by +2 solely for purposes of the roll to confirm a critical hit. The heartwire is so fine and light that it does not increase the armor’s weight.
Price: The market price modifier for such a suit of armor is +1,500 gp.

Source: Book of Exalted Deeds