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. 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, but this cannot be done nearly to the same extent as the Arrancar's technique. The strgenth of Hierro depends on the Espada and Arrcanar (Neliel and Ulquiorra being the examples, have much harder Hierro than the lower ranks of the Espada).