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K'mtar was an alias adopted by a future version of Alexander Rozhenko. In this alternate future, Worf was assassinated by political enemies, and Alexander, who had become a diplomat, was unable to save him. His efforts for peace was perceived as a weakness by other Klingons, causing Worf's assassination on the floor of the Council chamber.
He decided to travel to the past to 2370, and faked an assassination attempt on the Worf of that time, leaving evidence framing the House of Duras. Alexander then adopted the identity of K'mtar, gin'tak to the House of Mogh. He hoped that Worf would be sufficiently worried by the assassination attempt to send the young Alexander to a Klingon military academy, thus enabling him to protect his father in the future. When Worf stated that he would let Alexander make the choice, K'mtar threatened to invoke ya'nora kor, a bid to take custody of Alexander away from Worf. When this failed, he sneaked into Alexander's room, and attempted to kill him, knowing that this would end his existence, but save his father. Worf stopped him, and he confessed to Worf who he was and why he had returned from the future. Worf told him that with time now disrupted, anything could happen, and that things had changed. K'mtar was not seen again after this happened; his current status is unknown. (TNG episode: "Firstborn")