Dalek X's name was famous across the universe, but it was not his real name; he adopted the title in the knowledge that it evoked fear in humans. It was every bounty hunter's dream to defeat him, but also every bounty hunter's fear to encounter him. He was often called "the Devil in Dalek form". He commanded the first Exterminator-class Dalek vessel, the Exterminator, and had a squad of Assault Daleks equipped with laser-cutting claws in place of manipulator arms. He had at least two elite guards, armed with double gunsticks. It answered only to the Supreme Dalek. It ordered the destruction of a ship in the Auros escape fleet.
Dalek X lasted until the end of the Second Dalek War when he encountered the Tenth Doctor, travelling to the Dalek Base after the Doctor and a team of bounty hunters were captured. After enslaving them, he thought himself victorious over the Time Lord. He exterminated one of the Daleks in the Mines for progress being too slow, then gave orders that every hour the weakest team of miners should be exterminated. Dalek X tortured the Doctor in many ways, simply to see how much pain he could endure. In the end, his methods proved unsuccessful; the Doctor resisted them all. The Doctor offered him the TARDIS. He left with the Doctor and the surviving bounty hunters to the planet Hurala and the abandoned Lodestar refuelling station. In a rage, one of the bounty hunters fought the Daleks and was killed in the process, although the Doctor escaped with the others. Dalek X was attacked from below and knocked into a pit. The Doctor and the others detonated the station, destroying the other Daleks and the Exterminator, along with the other ships. The Dalek Fleet was thrown into disarray by this incident.
Dalek X was thought dead, but later the Doctor recognised his radio waves broadcasting across the universe for help, and homed in on him. The Doctor finished what he had started, leaving Dalek X to suffer forever in the darkness, living off life support systems and the astronic radiation, never able to get help because the planet's systems stopped other Daleks from receiving the signal and the planet had been quarantined due to the radiation, the Doctor claimed it would be at least 5000 years before anybody heard the transmission, by which time Dalek X would be gone. He did, however, promise to come back and get his revenge on the Doctor, who promised to be waiting for him. (PROSE: Prisoner of the Daleks)