Kastarion 3 was a planet where, during the 51st century, the Church was fighting a war, allegedly, against the Kastarions.
Astronomical data
Kastarion 3 was a barren, war-torn planet. At least five celestial bodies could be seen from a point of its surface, most notably a planet with very noticeable rings. The Fifteenth Doctor described Kastarion 3 as having "good trees, great mountains, rainbow crystals" and a very spectacular dawn, as well as being "basically empty", and wondered why anyone should fight on the planet.
Somewhere on the planet, the terrain was muddy and there was a large amount of fog. There was never snow there. Some of the Church clerics thought that the Kastarions, whom they believed they were facing in a war but never actually saw, were made of fog. Vacuum drones periodically flew above the warzone and drained the air of fog. Others, like Mundy Flynn, believed the Kastarions were in the mud.
In truth, Kastarions did not exist as a species, but were made up by the Clerics as the enemy after they inadvertently activated the Villengard Algorithm and started a war with no one. (TV: Boom [+]Steven Moffat, Doctor Who series 14 (BBC One and Disney+, 2024).)
History
On 5 October 5087, the Fifteenth Doctor stepped on a Villengard Smartmine, a type of land mine, and almost triggered it causing his own death as well as Ruby Sunday's, and the destruction of most of the planet. It was Ruby's first time ever visiting a planet other than Earth.
Earlier that same day, John Francis Vater was killed by a Villengard Ambulance on account of his blindness, leaving his daughter Splice Alison Vater an orphan. Canterbury James Olliphant was also later killed by an ambulance as the Doctor infected the Villengard Algorithm with a virus made by the reprogrammed courtesy posthumous AI partially containing the conscience of John Francis Vater. The virus also rendered the land mine the Doctor was standing on inoffensive.
The Doctor later concluded that there were in fact no Kastarions and that the land mine belonged to the Church themselves as part of a deceit by Villengard to keep selling more of their own warfare equipment. (TV: Boom [+]Steven Moffat, Doctor Who series 14 (BBC One and Disney+, 2024).)