The Klingon Second Dynasty was a period in Klingon history that occurred in the 16th century AD, and ended with Emperor Reclaw who was assassinated by General K'Trelan. All the members of the Imperial Family, including Shenara, were later put to death.
The next ten years that followed, the Empire was ruled by a council elected by the people, an era referred to as "The Dark Time" by Klingon historians. (DS9: "You Are Cordially Invited")
The Klingon Second Dynasty is not associated with, nor should be confused with the Second Empire.