Yartek

Yartek was the leader of the Voord.

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, dressed in the Keeper's robe and using the garment's cowl to hide his features, 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. (DW: The Keys of Marinus)

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