Armor of Fafnir

Blood Armor of the Evil Dragon. The continuously active Noble Phantasm of Saber of “Black,” Siegfried. Negates all attacks of B rank and below.
When receiving regular A rank attacks, the armor's defensive rank of B will be subtracted from the attack's offensive rank of A… in other words, it will be reduced to effectively an E rank attack, but in the case of an attack by a Noble Phantasm, the armor's defensive value will rise to the rank of B+. However, this additional defensive value will not be included in the attack calculations should the enemy use a Noble Phantasm or skill that has the properties of an Anti-Dragon special attack. Furthermore, the B+ defensive value is only applicable when he is attacked while defenceless, and the armor's defense will be further reduced should he take defensive action with “Balmung: Phantasmal Greatsword.” When he goes all out in a defensive battle, Siegfried is effectively a moving fortress.
However, since the effects of this armor do not apply to the single spot on Siegfried's back that was covered by the leaf of a lime tree, it becomes extremely difficult for him to be healed with healing magecraft if he is wounded there even once.

Fate/Apocrypha material: Fate/Apocrypha Encylopedia