A black hole converter was a device which placed on each of his warships. They were intended to destroy other planets and convert them into black holes when the New Time Lord Empire launched its war against the Universe. When he lost control of Earth, he told the Tenth Doctor he would rather destroy it, but the Doctor called his bluff, knowing that the Master would never kill himself. When time was reversed, the warships and black hole converter faded away. (TV: Last of the Time Lords)