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Challoner was a shapeshifter of unknown species, who was in charge of an organisation known as the Cosmic Peacekeepers. While he often took the form of a human, Challoner's true form was "lizard-like with slanting green eyes, long claws and dripping jaws".
In 1902, Challoner attempted to conquer the Earth by placing "temporal reversion" devices in all of the major capital cities outside of England, effectively making the British Empire the planet's dominant military force with him as their second-in-command under King Edward VII.
After forging documents from the Galactic Empire stating that the Earth was to be put under the control of the Cosmic Peacekeepers, Challoner hired a squad of Judoon to help him kidnap the King by extracting Balmoral Castle. When the Judoon realised they had been betrayed they withdrew their forces, leaving Challoner without allies.
In an attempt to save himself from the Tenth Doctor, Challoner took Martha Jones as a hostage. Martha managed to knock Challoner into his own temporal reversion machine, effectively erasing him from ever having existed. (PROSE: Revenge of the Judoon)