- For the primary universe counterpart, see Kahless the Unforgettable.
In the mirror universe, Kahless was a Klingon's mirror counterpart man 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. In the 24th century, the Terran's mirror counterpart archaeologist for hire Jean-Luc Picard's mirror counterpart located the Sword of Kahless's mirror counterpart and gave it to his Cardassian patron Gul Madred's mirror counterpart. (TNG - Mirror Universe novel: The Worst of Both Worlds)