Spiridon was a planet adopted by a base by the Daleks in the early 26th century as a part of their plan to destabilise the existing galactic balance of power prior to an invasion in force.
Astronomical and Geological Data
Spiridon's exact location was never revealed, but given the nature of the Dalek plan it seems likely it was in Mutter's Spiral, relatively close to the empires of Earth and Draconia.
Spiridon had an unsual geology in that its core was composed of molten ice. This would occasionally erupt onto the surface through fissures known as icecanoes.
Indigenous Life Forms
Non-intelligent Life
The planet was home to a number of non-intelligent predators, most of which were nocturnal. Many of the plant species also exhibited signs of sentience and were dangerous in their own right. One such species spat a mycogenic compound at anything which moved too close. The mould resulting from direct contact with this substance grew rapidly across the skin of anyone struck, rapidly incapacitating before eventually killing them.
Intelligent Life
The natives of the planet were known as Spiridons and were notable for being invisible. This was apparently an inherent characteristic, probably generated as part of essential life processes as it ceased to function post-mortem.
It is unclear whether the Spiridons had an additional compensatory sense or were actually invisible to each other as well (in which case their social interactions and reproductive behaviour must have been especially challenging). The former seems somewhat more likely as at least one native plant species had evolved the ability to detect invisible life forms.
When initially encountered by the Doctor the Spiridons had been enslaved by the Daleks and so the nature and level of their native culture prior to this is unknown. It did not appear to be more sophisticated than a primitive tribal society.
History
Nothing is known of Spiridon prior to the Dalek invasion, which occurred at some point prior to 2540. The Daleks established an extensive underground base near one of the icecanoes and subjugated the local natives.
The Daleks appear to have selected Spiridon as a base for two reasons: firstly, the icy core made it a promising location given part of their plan involved building up a massive army in sub-zero hibernation, although due to the erratic nature of the core artificial refrigerators were eventually used.
Secondly, the Daleks wanted to study the invisibility power of the native humanoids with a view to replicating it for themselves. Experiments met with some success although Daleks made invisible proved susceptible to light wave sickness which eventually incapacitated them. The Daleks on Spiridon were all said to have the power of invisibility, but none were ever seen using it (so to speak).
The Daleks' agent, the Master, was initially successful in fomenting tensions between Earth and Draconia, but disaster came from an unexpected quarter when the location of the Dalek base became known to the Thals, who dispatched several strike teams to stop the Dalek plan. The Thals were aided by the Doctor, who had learned of the Master's activities and been guided to Spriridon by the Time Lords.
The Daleks on Spiridon attempted to exterminate their enemies by releasing a lethal biological weapon to sterilise the planet. Sabotage by a Spiridon native led to the weapon being released within the Dalek laboratory before the base could be immunised against it.
The Doctor and the Thals were able to use explosives to trigger an icecano eruption, burying the army of ten thousand Daleks in molten ice and (they believed) permanent deep freeze. The Dalek Supreme was stranded on Spiridon after the base was destroyed and its ship stolen by the departing Thals (DW: Planet of the Daleks).
Some time later the Doctor brought Davros to Spiridon as part of his machinations to annihilate the Daleks permanently. Davros was able to revive Daleks from the frozen army and change their loyalties so they served him rather than the Dalek Emperor.
Daleks loyal to the Emperor on Skaro eventually came to Spiridon, led there by the Doctor, who was accompanied by Benny, Abslom Daak, and the other members of the Star Tigers. The loyalists were defeated by Davros' Daleks, with the almost-unnoticed assistance of some Spiridon natives.
Davros and his Daleks departed Spiridon to begin their conquest of Skaro, taking the Doctor and his friends with them. The planet was left in apparent peace after this. (DWM: Emperor of the Daleks).