The Dark Time
From Memory Alpha, the free Star Trek reference
According to 24th century "modern-day" Klingon historians, "The Dark Time" refers to the ten years following the fall of the Second Dynasty.
During this decade, the Klingon Empire was ruled by a council elected by the people. This was the first and only experiment in Klingon democracy known for producing several notable reforms. (DS9: "You Are Cordially Invited")
