Spiridon (also known as Zaleria) was the homeworld of the Spiridons. The planet was used for a base by the Daleks in the early 26th century in their plan to destabilise the galactic balance of power before an invasion in force.
When the Sixth Doctor initially thought he and Charlotte Pollard landed on Spiridon, he stated it was in the ninth system of "a far away galaxy", later called the "ninth galaxy" and later the "galaxy of Antares". (AUDIO: Brotherhood of the Daleks)
Spiridon had an unusual geology. Its core was composed of friable ice, called "molten". This would occasionally erupt onto the surface through fissures known as icecanoes. Natural springs also brought it to the surface, forming molten ice pools. The Plain of Stones absorbed heat during the day before releasing it at night. (TV: Planet of the Daleks)
The planet was home to several non-intelligent predators, most of which were nocturnal. Many of the plants, like the eye plants, also exhibited signs of sentience and were dangerous in their own right. One such species spat fungal spores at anything which came too close. The mould resulting from direct contact grew rapidly across the skin of anyone hit, rapidly incapacitating and eventually killing them. (TV: Planet of the Daleks)
The natives of the planet were known as Spiridons and were notable for being able to turn invisible. This was apparently an inherent characteristic, probably generated as part of essential life processes as it ceased to function post-mortem.
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. (TV: Planet of the Daleks)
Spearhead of Dalek invasion force
The Daleks selected Spiridon as a base for at least two reasons. The icy core made it a promising location, given part of their plan involved building a massive army in sub-zero suspended animation (although due to the erratic nature of the core which didn't give the Daleks the control they needed to maintain the army, artificial refrigerators were eventually used). The Daleks also wanted to study the invisibility of the native humanoids, with the intent to replicating it for themselves to use in their constant war efforts. Experiments met with some success, although Daleks made invisible proved susceptible to light wave sickness because they used too much energy. This incapacitated them and left them immobile. The Daleks on Spiridon were all said to have the power of invisibility, but only one was ever seen using it (so to speak), and was discovered to have died from light wave sickness. (TV: Planet of the Daleks)
The Daleks' agent, the Master, had been successful at first in fermenting tensions between Earth and Draconia, but disaster came from an unexpected quarter when the location of the Dalek base became known to the Thals. They dispatched several strike teams to stop the Dalek plan. The Thals were aided by the Third Doctor, who had learned of the Master's activities and been guided to Spiridon by the Time Lords. (TV: Frontier in Space)
The Daleks on Spiridon tried 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 used 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. (TV: Planet of the Daleks)
Site of battle between Dalek factions
The Dalek historian Caul Pornell believed that the lost Dalek army had been harvested by Davros to form the first ranks of the Imperial Daleks. (PROSE: Dalek: The Astounding Untold History of the Greatest Enemies of the Universe) This was in accordance with an account where the Doctor brought Davros to Spiridon as part of his machinations to annihilate the Daleks permanently. Davros revived Daleks from the frozen army and changed their loyalties so they served him rather than the Dalek Emperor.
A faction of Daleks loyal to the Dalek Emperor on Skaro, including a Special Weapons Dalek, came to Spiridon, led there by the Seventh Doctor, who was accompanied by Bernice Summerfield, Abslom Daak and the other members of the Star Tigers. The loyalists were defeated by Davros' faction, with the almost-unnoticed assistance of a few Spiridon natives. Davros had apparently spent some time (from his point of view) on Spiridon preparing for the confrontation.
Another account of the fate of the army existed. During the Second Great Dalek Occupation, the Daleks of the Seriphia Galaxy intended to awaken their lost army on this planet and recreate invisibility experiments for their conquest of Mutter's Spiral. The Seventh Doctor thwarted their plans and released the contagion that gave the Daleks a fatal light-wave sickness which Spiridons thought also killed the Doctor. The Dalek army was destroyed and invisibility was reclaimed to the natives of this planet. (AUDIO: Return of the Daleks)
No matter which account was correct, by the reign of the Restoration Empire, the Dalek army kept in hibernation in the Fifth Galaxy was regarded as the last force kept in stasis. (WC: The Sentinel of the Fifth Galaxy)
During the Time War, the Time Lords believed that the Spiridon army was still frozen. Recognising the threat of the Daleks excavating this army for use against Gallifrey, it was recommended that reconnaissance TARDISes make regular sweeps of the Spiridon system, and that any increase in Dalek activity on the planet be reported to the War Council immediately so that appropriate action ccould be taken. (PROSE: Dalek Combat Training Manual)
Spiridon was one of several planets where Daleks survived encounters with the Doctor. These survivors were classed as insane and taken to "intensive care" in the Dalek Asylum. The insane Daleks died when the asylum was destroyed by the Parliament of the Daleks with the help of the Eleventh Doctor, Amy Pond, Rory Williams and Oswin Oswald. (TV: Asylum of the Daleks)
Behind the scenes
The planet was originally called Destinus in the original scripts for Planet of the Daleks. (INFO: Planet of the Daleks) The Edge of Time depicts a timeline where the Dalek Empire stretches from Skaro to "New Spiridon". (NOTVALID: The Edge of Time)
Planet of the Daleks was included in the Time Lord Victorious compendium by James Goss after it was chosen to act as a prequel to The Sentinel of the Fifth Galaxy. This suggests the planet seen in that story is Spiridon, although nothing explicitly confirms this in the story itself.