It's different for everyone, you should have payed attention during the quest because it tells you word for word.
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A quest walkthrough for Demon Slayer can be found here: www.runescape.wikia.com/wiki/Demon Slayer
If this is part of the Demon Slayer quest, then the link below should direct you to your answer.
You have to do the demon slayer quest and near the end a person gives it to you to defeat the demon with it for silverlight. to get the blurite sword you have to do the knights sword. And you have to defeat the demon with the silverlight...
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In the Demon Slayer Quest on runescape you have to get the incantation for it at the start in Gypsy Aris's tent. She tells you that you need silverlight (the sword) and the incantation.