Chronotis

Salyavin was a Time Lord who was sentenced to be held on Shada, the prison planet of his people. He escaped, however, and lived on Earth as Professor Chronotis.

Prior to exile
Salyavin had great mental powers, and was able to "move his mind into another's", but was also able to make the Time Lords forget about Shada, partly by stealing a book called The Worshipful and Ancient Law of Gallifrey. Growing up, despite (or because of) Salvayin's criminal reputation, the Doctor admired Salyavin greatly.

As Chronotis
Salyavin secretly hid for centuries as a Human at St. Cedd's College in Cambridge and lived in his TARDIS, which was an earlier model than the the Doctor's TARDIS, which was a Type 40). The Doctor knew him and considered him a friend, though he did not know of the connection between Salyavin and Chronotis. (DW: Shada) The Doctor visited him several times over the decades, from his first incarnation onwards. (DWA: Cambridge Previsited)

When Salyavin began approaching his thirteenth incarnation, and thus the end of his cycle of regenerations, he sent a message to the Doctor to come and see him, so he could return The Worshipful and Ancient Law of Gallifrey, which he had stolen. However, it was not until the Doctor's eighth incarnation that he was able to follow up his request. (DW: Shada, WC: Shada)

He died when Skagra's sphere attacked him searching for information concerning The Worshipful and Ancient Law of Gallifrey, however due to a serendipitous mistake by Clare Keightley with Salyavin's TARDIS he was brought back into existence.

When Skagra visited Shada, he accessed the prison index and passed over Rungar and Sabjatric before he found Salyavin, who was imprisoned in perpetuity for mind crimes. Salyavin's full file read:

Salyavin - Mind Crimes Sec. 245/XR Sentence In Perp. Cabinet 9, Chamber T

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. (EDA: Unnatural History)
 * Douglas Adams would reuse the character of Professor Urban Chronotis in his novel Dirk Gently's Holistic Detective Agency without all elements copyright of the BBC. For the radio adaptation of the novel, Chronotis was played by Andrew Sachs, who had also appeared in several Doctor Who stories.