Dalek X

Dalek X was one of the most sacred and feared Daleks before the Last Great Time War. He is described as having a gunmetal black casing with gold vertical slats and sensedomes.

His name was told across the universe and it was every bounty hunter's fear to encounter him, yet every bounty hunter's dream to defeat him. He lasted until just before the Time War when it met the Doctor. After enslaving the Doctor and a team of bounty hunters, he thought himself victorious over the Time Lord. He allowed the Doctor to take him back to the planet, where the Doctor led Dalek X into a room about to explode, then escaped in his TARDIS, taking the surviving bounty hunters with him.

Dalek X was thought to die in that room, although later the Doctor recognized his radio waves broadcasting across the universe for help and so homed in on him. The Doctor finished what he had started, leaving Dalek X to suffer forever in the darkness, living off of life support systems, never being able to find help as the planet's systems stopped fellow Daleks from receiving the beacon. (NSA: Prisoner of the Daleks)