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In the mirror universe, the Sword of Kahless was a bat'leth sword used by the legendary Klingon warrior Kahless in ancient times.
His sword, which survived him, became a valuable artifact but was lost for centuries. At some point prior to 2371, the Terran archaeologist for hire Jean-Luc Picard located the Sword of Kahless and gave it to his Cardassian patron Gul Madred. (TNG - Mirror Universe novella: The Worst of Both Worlds)