Hierro

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Hierro (鋼皮（イエロ）, hiero; Spanish for "iron") is the special skin of Arrancar where an Arrancar's spiritual energy condenses and creates a steel hard skin that is strong enough to block a Zanpaktou. Also depending on the Arrancar's power level the stronger the Hierro, Noitora Jiruga claims to have the strongest Hierro despite being ranked number 5, while technically Ulquiorra, Barragan, Stark and Halibel would have the strongest Hierro