If this is part of the Demon Slayer quest, then the link below should direct you to your answer.
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A quest walkthrough for Demon Slayer can be found here: www.runescape.wikia.com/wiki/Demon Slayer
look on runehq
By doing Demon slayer quest
Talk to Gypsi Aris and she will give you it.
Talk to Gypsi Aris and she will give you it.