A Klingon chancellor of the Second Empire who sent a fleet to conquer the Breen homeworld, none of which ever returned, a cautionary tale Worf recalls as a Breen prisoner.
Mow'ga was chancellor during the period Klingon historians call the Second Empire. Determined to add the Breen to his conquests, he dispatched an invasion fleet against their homeworld. The ships were never heard from again, and the episode left Klingons with a lasting wariness of the Breen.
Worf tells the story in the final arc of Deep Space Nine, in 'Til Death Do Us Part, while he and Ezri Dax are held aboard a Breen vessel after being seized on Goralis. It is one of the few things anyone on the station can say about the secretive species before the Breen announce their alliance with the Dominion.
Mow'ga is never seen, and nothing else about his reign is established.