The Klingon Third Dynasty is a period in Klingon history that occurred in the 16th century CE, that was founded following what was known as "The Dark Time".
Because the true Imperial bloodlines were cut at the end of the Second Dynasty, a new group of Klingons were given the titles and names of the original Imperial Family in order to create the illusion of an unbroken line. (DS9: "You Are Cordially Invited")