For the primary universe counterpart, see Kahless the Unforgettable.

In the mirror universe, Kahless was a Klingon male who lived in ancient times. After his death, he was known to the Klingon people as "He Who Shall Return." The Regent of the Klingon Empire held the throne in anticipation of Kahless' return to lead the Klingon Empire. (TOS - Mirror Universe novel: The Sorrows of Empire)

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)