The bow-sword is required in the Devious Minds quest, for the monk at the Paterdomus temple on the way to Canifis. Although the bow sword offers a wield option, attempting to do so produces an exclamation that "It's a bow and a sword...how am I supposed to use this?!" Asking the monk to teach the player how to use it produces the comment that "it's a foreign weapon" and "perhaps later" one of their warriors can teach them.
To make the bow-sword, you need to use a Mithril two-handed sword on Doric's whetstone, directly east of Taverley to produce a Slender blade. To finish it off, players need to add a bowstring to the sword.
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