For the regular universe counterpart, see Kahless the Unforgettable.

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

His sword, which survived him, became a valuable artifact. In the 24th century, Jean-Luc Picard located the Sword of Kahless and gave it to Madred. (TNG novel: The Worst of Both Worlds)