Commander Kaagh also called Kaagh the Slayer, was attached to the Tenth Sontaran Battle Fleet's Special Assault Squad. He was the lone survivor of a failed invasion of Earth by the Sontarans. Also, he was the first Sontaran to be known to act on the behalf of another species, serving the Bane as a mercenary.
Luke Rattigan chose to istract Professor Nicholas Skinner and put them under his control with neural control implants; he wanted the Professor to act solely on his orders, while Lucy arrived to investigate the "lights" in the sky. These were actually drones Kaagh had used to lure out the Skinners.
Kaagh found the consciousness of Horath and used Kaagh as a go-between to become emperors of the galaxy.
Mrs Wormwood travelled to Earth, closely followed by Kaagh, who came in during a siege by the Bane Kindred. He found Mrs Wormwood and killed her attackers. They stole Luke and the Tunguska Scroll, which would show them where Horath was buried. Luke escaped, but Kaagh caught him. However, Mrs Wormwood was growing tired of him.