Spiridon (species)

The Spiridons were the dominant species of sentient humanoids on planet Spiridon in the Ninth System. They had developed a form of invisibility but became visible after death. They were eternal beings, never dying of old age, however they could be killed with any other means. They had been subjugated, to be used to be experimental subjects and slaves, by the Daleks who were attempting to discover the secret of the Spiridons' invisibility and reproduce it for their own use. Some of the Spiridons, including one called Wester, resisted. They wore furs to keep themselves warm.(DWPlanet of the Daleks)

The Doctor returns to Spiridon in spin-off audio adventures Return of the Daleks and Brotherhood of the Daleks