A recap of the long, convoluted history of the dagger that killed the Night King:
https://www.nytimes.com/2019/04/29/arts/television/arya-dagger-bran-night-king.htmlIt first appeared in the hands of the assassin who tried to kill Bran way back in Season 1 Episode 2, and then passed to Catelyn Stark, Ser Rodderick Cassel, Littlefinger, Ned Stark, then Littlefinger again, who eventually gave it to Bran as a gift. Bran gave it to Arya, who later used it to kill Littlefinger, as well as the Night King.
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