Hierro

is a technique of the Arrancar in which an Arrancar's spiritual energy 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 with difficulty. It is unknown if Nnoitra's claims are true as four Espada are of higher rank and power.