Kastarion 3
Kastarion 3 was a planet that, according to the Fifteenth Doctor, was uninhabited. The Doctor described the planet as having "good trees" and "great mountains", and rated it a "seven out of ten". (TV: Boom [+]Loading...["Boom (TV story)"])
History
In 5087, when the Anglican Marines landed on the planet, they fired warning shots to advertise their presence as standard procedure. That act activated the Villengard algorithm, effectively declaring war on an empty planet. The marines didn't realise that the planet was empty, and as the algorithm kept the one-sided war running, they assumed they were fighting unseen natives that they called "Kastarions".
Six months later, the Doctor and Ruby Sunday landed in a battlefield, and upon hearing John Francis Vater's scream as the Villengard ambulance euthanised him, the Doctor ran to help, inadvertently stepping on a Villengard landmine. An AI simulacrum of Vater was sent by the Doctor as a computer virus into the Villengard algorithm to find proof that the planet was, in fact, uninhabited, but the simulacrum disabled the algorithm entirely, disabling the landmine in the process, and ending the conflict. (TV: Boom [+]Loading...["Boom (TV story)"])
