Spiridon was a jungle planet that was home to a naturally invisible race of aliens known as the Spiridons. The Daleks once tried to invade Spiridon to learn the secrets of invisibility, but they were stopped by the Doctor. Since then, however, the Daleks have shown an interest in Spiridon and hid many bases there. They also concealed an army beneath the surface of Spiridon, which was entombed in molten ice by the Doctor later on.
During the Dalek Civil War, the Imperial Daleks owned Spiridon but were driven out by the natives due to their thinned numbers. Since then, the Daleks have avoided Spiridon and it seems to have entered their legends as a taboo planet, rather like Earth, and that any attempts of an invasion will always end in disaster.