Carnathon

was once taken to a planet called Carnathon to be executed. (TV: Extremis, PROSE: Missy)

A stipulation of the Fatality Index was that the execution of a Time Lord should be carried out by another Time Lord. As they were "not easy to come by", the Twelfth Doctor was chosen. (TV: Extremis)

Behind the scenes

 * While the planet is left unnamed in TV: Extremis, PROSE: Missy gives the planet's name as Carnathon.