Voord

The Voord are vicious featureless creatures clad in acid-resistant suits who, having developed a resistance to the conscience of Marinus, invaded the pyramid housing, killing Arbitan (the keeper of the Conscience of Marinus, and he forced the Doctor and his companions to search for the four keys to the conscience. He was later killed by the Voord) in their quest to get the four keys.

The Voord was only in The Keys of Marinus.