Skull Moon was a moon and the site of a battle in the Last Great Time War, (PROSE: The Day of the Doctor) the Battle of Skull Moon. (PROSE: Dalek Caan) The War Doctor took command in the slaughter of Skull Moon. (PROSE: The Day of the Doctor) Gastron was among the Time Lord soldiers who fought with him. (TV: Hell Bent)
The humanoid gunship pilot Cass Fermazzi was present at Skull Moon and "wept at the massacre". (PROSE: The Day of the Doctor)
As recorded by the Testimony, the "Butcher of Skull Moon" was a name for the Doctor ever after. (TV: Twice Upon a Time)