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The Heartslicers were the twin swords of the Hairy Scary Librarian. The Librarian was a skilled swordsman, and notorious for using these swords to kill anyone caught in the Meathead Public Library without permission.
In A Hero's Guide to Deadly Dragons, the Hairy Scary Librarian attacked Hiccup Horrendous Haddock III, Camicazi, and Fishlegs No-Name when he discovered them in the library, and was able to duel Hiccup and Camicazi simultaneously (though he was put out at his inability to finish them quickly).
In How to Seize a Dragon's Jewel the Librarian called his amber nets Heartslicers.