Cimmeria IV was a planet in the Cimmerian System enveloped in total darkness. The surface of the planet was very rocky and bitterly cold, due to the lack of a sun's light to heat it up. Throxillian explorers and miners set up a base there, intending to exploit its apparent lack of indigenous life for commercial purposes.
A system of mini-suns were rigged up in orbit to light the planet. Their activation was stopped when the Cimmerians living on the planet blanketed the base in particles that disrupted electronic circuits, disabling the system. The particles also burned out the eyes of the base personnel.
The Eighth Doctor mediated the subsequent conflict between the Cimmerians and the ROSM rescue ship, and activated the orbital sun system. Shortly afterward he learned that the Cimmerians had kept the planet in darkness to hide from their supposed persecutors, the Solarians. With the suns active, the Solarians' solar sail ships found the planet and the two groups reconciled an ages-long misunderstanding. (AUDIO: Embrace the Darkness)