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)

While pondering whether he should try to find out the Enemy's true identity, the Eighth Doctor once reflected that the Enemy's identity would probably be dreadfully disappointing, and cited the Enemy being "Yartek, leader of the alien Voord, carrying a big stick" as one such guess. (PROSE: The Taking of Planet 5)