Chronotis

Professor Salyavin, dubbed the Great Mind Outlaw by Gallifreyan historians, was an infamous Time Lord criminal. Condemned by his fellow Gallifreyans for his unethical use of the uniquely powerful mind-control abilities he possessed, he escaped Shada, created himself a new identity as Professor Chronotis, and eventually retired to Earth, taking with him the The Worshipful and Ancient Law of Gallifrey. Despite his raucous past, Chronotis gave everyone who met him the strange feeling that "he seemed like such a nice old man".

As Salyavin
Salyavin was born over ten thousand years before the Doctor's time. (PROSE: Shada)

Like all Time Lords, Salyavin was taken from his family at the age of eight for the selection process in the Drylands. Staring into the Untempered Schism as part of a Time Lord initiation rite, Salyavin was said to have been driven mad by what he saw in the Schism by one Time Lord historian's account. (PROSE: A Brief History of Time Lords)

He had great mental powers, being able to move his mind into another being's. (TV: Shada) According to the life story he would later relay to the Fourth Doctor, he had no malicious intent and simply wished to become a respected academic, but the High Council was suspicious of him and his powers, fearing he might try to use them to take over Gallifrey. These fears were supported by a few amusing, but harmless "pranks" he played on high-ranking Time Lords in his younger days using his power. As he continued to climb through the ranks of Gallifreyan hierarchy, the Council decided to trick him, sending him to Shada as an official and planning to trap him inside forever. The Council then began a supposedly-posthumous smear campaign on Salyavin, telling of him as a crazed villain, soon nicknamed the Great Mind Outlaw. (PROSE: Shada)

According to another account, the Master, in his first incarnation, gained the trust of Salyavin, one of his Professors at the Time Lord Academy, in an effort to gain access to the restricted libraries of Gallifrey and find a very dangerous book. The Master never found the book, but the affair left Salyavin implicated. Condemned to imprisonment on a certain prison planet, and bitter at the false accusation, he decided to actually steal the book, just to spite the High Council, and took it away with him as he was carried away to Shada, though with every intention of escaping. (PROSE: The Legacy of Gallifrey)

Salyavin instead took control of his guardians, planting into their mind the idea that they had already locked him up before sending them on their way. Although he had arrived to Shada in his guardians' TARDIS, he had brought his own, disguised by its chameleon circuits as a bookmark, allowing him to escape the prison planet. Fearful that his escape might be noticed, Salyavin committed what he later described as the least moral use of his powers he had ever dared, linking himself to his own TARDIS's telepathic circuits to be able to broadcast his mind all over Gallifrey. From this position of unparalleled psychic power, he was able to erase all memory of Shada from the Time Lords, thus ensuring no one would ever return there and witness that Salyavin's cell was empty. The physical stress of this feat nearly killed Salyavin, forcing him to regenerate into a new body.

Taking advantage of the transformation, he returned to Gallifrey with the brand-new alias of Professor Chronotis, a harmless archivist — a persona he retconned into all relevant officials' memories with one more use of his mind powers. As curator of the Panopticon Library, he kept a close eye on the book The Worshipful and Ancient Law of Gallifrey, one of the Artefacts of Rassilon and nothing less than the key to Shada, and thus the ruse that Salyavian was gone and Chronotis innocent was complete. (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. After taking The Worshipful and Ancient Law of Gallifrey with him from the archives and leaving a harmless decoy in its place, he elected to move to St Cedd's College in Cambridge around the 17th century (PROSE: Shada) and lived in his TARDIS, rescued "quite literally" from the scrap heap and disguised as his living quarters at the College. Under Time Lord law, an officially retired Time Lord was not allowed to own or possess a TARDIS. (TV: Shada), PROSE: Shada) He had already lived at the college for some time by 1889. (COMIC: The Time Machination)

Chronotis met the the Doctor in 1955 while he was in his fourth body and travelling with Sarah Jane Smith. A few years later, the First Doctor met Chronotis for the first time. (PROSE: Cambridge Previsited) The Doctor grew to know him as a friend, though he did not know of the connection between Salyavin and Chronotis. (PROSE: Shada)

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. (WC: Shada, TV: 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 Chronotis was Salyavin. Due to a serendipitous mistake made by Clare Keightley with Salyavin's TARDIS, he was brought back into existence. (WC: Shada, TV: 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, Salyavin returned to Cambridge to live as Professor Chronotis. (PROSE: Shada)

Behind the scenes

 * After the BBC's failure to complete Shada, writer Douglas Adams reused his creation of Professor Chronotis (whose rights he owned) in his novel, there revealing his first name to be "Urban". Though Chronotis had been created for Shada, this makes the novel his first appearance in terms of release. Of course, this had to be achieved without elements copyrighted by the BBC; as such, while he remains an absent-minded professor of St Cedds who has lived for hundreds of years, the book's Chronotis is not stated to be a Time Lord. He does travel through time, but as a TARDIS could not be explicitly featured, Chronotis's time travel is steered not through the familiar control panel but by using an abacus to plot his route through the continuum — a functionality he seems to lose when his telephone line is repaired (Chronotis's lack of a working telephone line is alluded to in Gareth Roberts's later novelisation of Shada). Chronotis does state that he "retired" to Cambridge, but cannot remember what he retired from (this being, per Shada, being a Time Lord).
 * In John Leekley's ultimately-unproduced version of Shada, Chronotis would have been introduced as Romana's uncle. (REF: The Nth Doctor)