Sebastian Malvern was a human psychic. He lived in Malvern Hall, built in 1649 and part of his family's estate.
The Cylox Tko-Ma established contact with Malvern and renamed him Kyto-Ma, using him as a link to the Earth's plane of existence. They created the Magnate, a secret organisation whose true purpose was to find Tko-Ma's brother Lai-Ma.
Malvern did not completely trust Tko-Ma, though. When he found out that the Cylox planned to continue the crimes he had been imprisoned for if he was released from his other dimensional jail, he helped the Sixth Doctor re-imprison him. He also repaired as much of the mental damage as possible done to the teenagers who had been made into Nestene drones.
On a psychic power scale where the Doctor was 6 and Trey Korte a 7, Malvern was a 50. (PROSE: Instruments of Darkness)