The Changeling Martok was a Changeling of the Founders in the 24th century. In the 2370s, this changeling was one of a handful agents tasked with destabilizing relationships between the larger organizations of the Alpha Quadrant.
In mid-2371, this changeling replaced General Martok while hunting saber bear along Kang's Summit on Qo'noS. Following his capture, the real Martok was sent to Internment Camp 371. (DS9: "In Purgatory's Shadow") The changeling's mission was to increase hostilities with the Cardassian Union and United Federation of Planets in order to weaken each organization. (DS9: "The Way of the Warrior", "Apocalypse Rising")
In early 2372, the changeling convinced Chancellor Gowron that a recent coup on Cardassia Prime was effected by the Dominion. After leading a fleet to Deep Space 9, the Klingons invaded Cardassian space. The Martok Changeling also attempted to incite conflict between the Federation and the Klingon Empire; to this end, he had Kaybok, one of his commanders, executed for refusing to fire on the USS Defiant.
Martok and Gowron later led the Klingons in an attack on Deep Space 9. He attempted to continue the battle even when the odds turned against the Klingons, but was overruled by Gowron. Nevertheless, the incident led to a year-and-a-half conflict between the Klingon Empire and the Federation in a conflict lasting from 2372 to 2373. (DS9: "The Way of the Warrior")
In early 2373, Martok attended the induction of Order of the Bat'leth members on Ty'Gokor. He recognized Benjamin Sisko, Miles O'Brien, Worf and Odo, who were there undercover to expose Gowron as the changeling. Martok imprisoned them and attempted to fool them into killing Gowron instead, thereby gaining power for himself and furthering the war. However, Odo discovered him to be a Founder and drew him into exposing himself in front of the crowd. The changeling was promptly killed by a hail of disruptor fire. (DS9: "Apocalypse Rising")
The real Martok was informed of the Martok Changeling's actions by Julian Bashir, Worf and Elim Garak at the internment camp later that year, remarking it was "a grave dishonor". (DS9: "In Purgatory's Shadow")