Killers of the Dark, also known in fan circles as The Killer Cats of Geng Singh, was an unproduced six-part serial written by David Weir that was to have concluded Season 15. Graham Williams decided another Gallifrey story needed to be written after the success of The Deadly Assassin and commissioned Weir with the task. It would feature Cat-People native to Gallifrey who would live in a culture similar to that of many Asian countries. It would also feature a scene set in a gladiatorial arena, filled with Cat-People. When run past script editor Anthony Read and director Gerald Blake, it was cancelled as they believed it would be too hard to film. Williams and Read then wrote The Invasion of Time to replace it.