Chronotis

Salyavin was a Time Lord sentenced to be held on Shada, the prison planet of his people. He escaped 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". He made the Time Lords forget about Shada, partly by stealing the book The Worshipful and Ancient Law of Gallifrey. Growing up, despite (or because of) Salvayin's criminal reputation, the First Doctor admired Salyavin greatly. (DW: Shada)

As Chronotis
Wanting to forget his hateful powers, Salyavin hid for centuries as a human at St Cedd's College in Cambridge and lived in his TARDIS, a model earlier than the Doctor's Type 40, disguised as his living quarters at the College. The Doctor knew him as 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. (BE: 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. However, it was not until the Doctor's eighth incarnation that he appeared. (DW: Shada, WC: Shada)

Salyavin died when Skagra's sphere attacked him, searching for information about The Worshipful and Ancient Law of Gallifrey. Due to a serendipitous mistake made 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 Sabjatrik 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 (WC: Shada)

Fate
Like the rest of the Time Lords, Chronotis likely would have died in the Last Great Time War. Despite this, however, the Tenth Doctor spoke of him as if he was still alive at Cambridge. (IDW: The Time Machination)

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 Dirk Gently's Holistic Detective Agency without elements copyright by the BBC.