The Mentiads were a group of individuals native to Zanak which 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.
After the Doctor, Romana and K-9 interceded and assisted them, the Mentiads destroyed the Captain's bridge and Xanxia's time dams. (DW: The Pirate Planet)