Yartek

Yartek was the leader of the Voord expedition to Marinus.

Biography
The Monk once worked as an advisor to Yartek. (PROSE: No Future)

Yartek discovered how to resist the Conscience of Marinus. Invading the building where the Conscience was kept, Yartek and his followers killed Arbitan. Yartek then posed as Arbitan — dressing in the Keeper's robe, keeping in the shadows and hiding his features beneath the hood of the garment — to get the keys to the Conscience from Ian Chesterton. He was killed in the destruction of the Conscience when he placed a false key in the machine resulting in an explosion. (TV: The Keys of Marinus)

A hundred years later, Tarlak, another Voord leader who tried to conquer Hydra, described Yartek as his brother. (AUDIO: Domain of the Voord)

The Eighth Doctor jokingly claimed that the Enemy was "Yartek, leader of the alien Voord, carrying a big stick". (PROSE: The Taking of Planet 5)