The Mentiads were a group of individuals native to Zanak. They had gained psychic powers following the destructions of the populations of the planets that Zanak destroyed in pursuit of rare minerals for Xanxia's time dams. They absorbed the psychic energy released when Zanak's mining equipment destroyed the matter of the planets it mined. As each new Mentiad was "born", their powers added to their fellows to create a gestalt mind, but their abilities were limited by their inability to understand the Captain's evil due to their essentially good nature.
After the Fourth Doctor, Romana I and K9 interceded and assisted them, providing them with the leader and focus that they required to channel their powers effectively against their enemies, the Mentiads destroyed the Captain's bridge and Xanxia's time dams. (TV: The Pirate Planet)