Hierro

is a technique of the Arrancar in which an Arrancar's Reiryoku condenses and creates a steel-hard skin that is strong enough to block a Zanpakutō. Depending on the Arrancar's power level, their Hierro may be stronger; Nnoitra Jiruga claimed to have the strongest Hierro and could be cut by Kenpachi Zaraki only after he adapted to the former's Reiatsu. It is unknown if Nnoitra's claims are true as four Espada are of higher rank and power, and a former Espada was able to injure him without reciprocation.

The technique is exclusive to Arrancar, though it is yet to be seen if Vizard are capable of creating Hierro during their hollowification. Shinigami have been shown using Reiatsu to block or lessen the blows of Zanpakutō. For example, when Ichigo Kurosaki used his reiatsu to momentarily stun Shūsuke Amagai. Also, when Ichigo and Kenpachi Zaraki first fought, Ichigo couldn't land a single scratch on him. It wasn't until the apparition of Zangetsu appeared that he begin to put more power into his attacks and regain his resolve that he managed fight on even grounds with Kenpachi. However, this ability cannot be done nearly to the same extent as the Arrancar's technique.

Ichigo, in his half-transformed Hollow state exhibited a variation of this technique where, after enduring several injuries inflicted upon him by the vizard, his wounds immediately regenerated into bone-like carapaces that appear to be just as resistant as the standard Hierro, if not more.

Trivia

 * Although all the Arrancar have the feature of Hierro, there is a good possibility that some don't benefit from it as much as the others do. The Octava Espada, Szayel Aporro Granz was shown being able to bleed from even the slightest attacks; an occurance that wouldn't have affected any of the other Arrancar.