Cessair of Diplos, also known on Earth by a variety of aliases, was a long-lived fugitive criminal.
Cessair was wanted for multiple counts of murder and for the theft of the Great Seal of Diplos (which was actually the third segment of the Key to Time). She escaped Diplos and took three Ogri from Ogros (which contravened Article 7594 of the Galactic Charter).
She and her servants were eventually captured. They were to be returned to Diplos with the Megara, sentient justice machines. En route, Cessair escaped, took control of the ship and diverted it to Earth. She hid on the primitive planet, her starship hidden in hyperspace.
Cessair hid on Earth for four thousand years, adopting a number of identities. The earliest one was the Celtic war goddess known as the Cailleach. In the Middle Ages, she was the Mother Superior of the Convent of the Little Sisters of Saint Gudula. The convent was later destroyed during Henry VIII's dissolution of English monasteries. In 1572, Boscombe Hall was built on the grounds of the old convent, and Cessair lived there for the next four centuries.
After the 18th century, she was known variously as Lady Morgana Montcalm, the reclusive Mrs. Trefusis who lived there alone for sixty years and the widowed Señora Camara whose husband supposedly did not survive the trip from Brazil. When the Fourth Doctor and Romana I met her in 1978, she was going by the name Miss Vivien Fay. Despite the changes in identity, she always controlled the land around the stone circle where the Ogri rested and where, in hyperspace, the starship waited.
On the Fourth Doctor's quest to find the segments of the Key to Time, the tracer led the TARDIS to Earth. Romana was kidnapped by Cessair and brought to the starship in hyperspace. The Doctor followed her, but accidentally released the Megara, who sentenced him to death.
After a gruelling trial, the Doctor made the Megara realise Cessair's true identity. Having established her guilt, they sentenced her to perpetual imprisonment and transformed her into another stone in the circle. (TV: The Stones of Blood)