K'Trelan

From Memory Alpha, the free Star Trek reference.

K'Trelan was a Klingon general who served under Emperor Reclaw during the Second Dynasty. K'Trelan assassinated Reclaw and brought the Second Dynasty to an end, installing in its place a democratically elected government. This period would later be referred to by Klingon historians as "The Dark Time". (DS9: "You Are Cordially Invited")