Klingon Second Empire

From Memory Alpha, the free Star Trek reference.

The Klingon Second Empire was lead by Chancellor Mow'ga.

During his reign, he ordered a fleet of Klingon warships to invade and conquer the Breen. The fleet was never heard from again. (DS9: "'Til Death Do Us Part")

The Klingon Second Empire is not associated with, nor should be confused with the Second Dynasty. It is possible the First and the Second Empire were divided by the Hur'q invasion, which would be comparable to the "First" and "Second French Republic" or the "First" and "Second German Empire" in Earth history.