The pronoun "it" refers to the word "iron" which refers to a weapon.
As you may already know, the pronoun "it" refers to an inanimate object or animal. Like all pronouns, this pronoun has the function of:
In the case of the text above, we can see how this pronoun replaces the noun "iron" presented in the first line. This noun refers to a weapon, which has had a lot of battle experience and bears many marks.
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