A Rat Emperor, or Rattenkaiser, was encountered by the Seventh Doctor and Ace when they visited the planet Epajaenda. It was a giant globe of rats which attacked the Travellers after several rats were killed by the Traveller's guards. It killed the guards, but was scared away by the arrival of Abraham Derris-Cuthbertson's spaceship. The Rat Emperor later returned and engulfed Derris-Cuthbertson, but he managed to open the anomaly, channelling Dirrijali's oceans onto Epajaenda and drowning the Rat Emperor. (PROSE: Last Rites)
Tardis