The Martok Changeling is a villainous founder of The Dominion who posed as the Klingon war hero Martok in the fourth and fifth seasons of Star Trek: Deep Space Nine.
He was portrayed by J.G. Hertzler.
In mid-2371, Martok, a highly respected general in the Klingon Empire, was replaced by a Changeling while hunting saber bear along Kang's Summit on Qo'noS. Following his capture, the real Martok was sent to Internment Camp 371.
In early 2372, the Changeling Martok tried to weaken the Alpha Quadrant by pushing Chancellor Gowron into believing that the coup on Cardassia Prime was influenced by the Dominion. After leading a fleet to Deep Space 9, the Klingons invaded Cardassian space. The Martok Changeling, even at this stage, was also hoping for a conflict between the Federation and the Klingon Empire; to this end, he had one of his commanders (Kaybok) executed for refusing to fire on the USS Defiant.
The Martok Changeling and Gowron later led the Klingons into battle against Deep Space 9, that eventually led to a year-and-a-half conflict between the Klingon Empire and the Federation in a conflict lasting from 2372 to 2373.
In early 2373, Martok was discovered to be a Founder by the disguised Odo, and was killed by a hail of disruptor fire at Ty'Gokor when discovered.
The real Martok considered all the deaths his Changeling duplicate caused a source of shame. He was rescued from imprisonment by Worf and Garak and remained on Deep Space 9 as commander of a Klingon military detachment. He would become a friend to much of the crew, and soon invited Worf to join his House in order to restore his honor amongst other Klingons.