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 when he discovered them in the library, and was able to duel Hiccup and Camicazi simultaneously (though he was frustrated at his inability to finish them quickly).
Madguts the Murderous confiscated the Librarian's swords when he took the Librarian as a slave to the Ugli-thug Slavelands, in How to Seize a Dragon's Jewel. The Librarian called his amber nets 'Heartslicers', in honor of his two swords.
The Heartslicers are weapons used by the Hairy Scary Librarian, which he uses primarily to defend the Meathead Public Library.