Yartek

Yartek was the leader of the Voord expedition to Marinus.

Biography
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)