Skonnos

Skonnos was a brown, dirty planet with a grey sky. This may have been caused by the Skonnans during their civil war or the Nimons. The planet was ruled by the first Skonnan Empire, but it collapsed due to infighting until only the army was left. Eventually, they were then discovered by the Nimons, who bribed them with advanced technology. Though the Nimons planned to use the energy of Skonnos themselves, they were stopped by the Fourth Doctor, Romana II, K9, Seth and Teka. (TV: The Horns of Nimon)

According to one account, Koschei before he became the Master, arrived on Skonnos at the height of the first Skonnon Empire. (PROSE: The Dark Path)