Spiridons
The Spiridons, also known as Valerians, were the invisible humanoids that were native to the planet Spiridon.
Biology
The Spiridons were naturally invisible at all times thanks to anti-reflecting light. This ability made them targets of the Daleks who wanted to use this ability for themselves. The high energy demands and the risk of light wave sickness made the invisibility difficult to duplicate in other species.
Once a Spiridon died, they would become visible They could also make themselves visible by undergoing extensive, painful treatments by regularly ingesting certain chemicals which added pigmentation and caused cellular decay.
Appearances
Planet of the Daleks (TV)
Emperor of the Daleks! (Comic)
The Scarlet Empress (Prose)
Return of the Daleks (Audio)