Chronotis

Salyavin was an infamous criminal Time Lord, who used his unique power of mind-control to make others do his will. Despite being a dangerous and wanted criminal, the Doctor grew up looking up to the man. After being caught by his own species, he was sentenced to be held on Shada, a prison planet. He escaped shortly into his sentencing and took on the fake personal Professor Chronotis. When he moved from Earth to Gallifrey to retire, he took with him the The Worshipful and Ancient Law of Gallifrey, which was the key to finding the prison, so that no one could ever find it.

As Salyavin
Salyavin lived over ten thousand years before the Doctor's time. He had great mental powers, and was able to move his mind into another's. Though he had no malicious intent, the High Council was suspicious of him and his powers. Though he continued to move up the ranks, he simply wanted a quiet life of reading. The Council decided to trick him, sending him to Shada as an official and planning to trap him inside forever.

Salyavin instead took control of his guardians, allowing him to escape. He then used his powers to make Salyavin into a myth and changed his identity to Chronotis. He also removed the knowledge of Shada from their minds, as well stealing the book The Worshipful and Ancient Law of Gallifrey. (PROSE: Shada)

As Chronotis
Chronotis lived a quiet life, acting as a librarian for nearly 12,000 years. Eventually, for his years of service, the Time Lords allowed him to retire to another planet. He decided to move to St Cedd's College in Cambridge around the 17th century and lived in his TARDIS, disguised as his living quarters at the College. (PROSE: Shada) He had lived at the college for some time by 1889. (COMIC: The Time Machination) The Doctor knew him as a friend, though he did not know of the connection between Salyavin and Chronotis. (HOMEVID: Shada) The Doctor visited him several times over the decades, from his first incarnation onwards. (PROSE: Cambridge Previsited)

When Salyavin began approaching his thirteenth incarnation, and thus the end of his cycle of regenerations, he sent a message to the Fourth Doctor to come and see him so he could return The Worshipful and Ancient Law of Gallifrey, which he had stolen. (HOMEVID: Shada, WC: Shada)

At the same time, Skagra was looking for a way into Shada and access to Salyavin. Combining a mind stealing sphere and Salyavin's ability to implant his mind, Skagra planned to merge minds with everyone in the universe. (PROSE: Shada) Salyavin died when Skagra's sphere attacked him, searching for information about The Worshipful and Ancient Law of Gallifrey. Skagra didn't realise that Chrontis was Salyavin. Due to a serendipitous mistake made by Clare Keightley with Salyavin's TARDIS, he was brought back into existence. (WC: Shada) Skagra eventually made it to Shada, where he found out that Salyavin wasn't there. Unfortunately, Chris Parsons worked out who Salyavin was while he was in the room, allowing Skagra to take over his mind. After the Doctor defeated Skagra, Salyvin returned to Cambridge to live as Professor Chronotis. (PROSE: Shada)

Behind the scenes

 * It is suggested in various readings that Salyavin also appears in the Eighth Doctor Adventures novel Unnatural History as Professor Daniel Joyce, a professor at Berkeley. In the novel, the Eighth Doctor seeks his assistance while in San Francisco. This was later denied by author Jonathan Blum.
 * Douglas Adams would reuse the character of Professor Urban Chronotis in his novel  without elements copyright by the BBC.