Professor 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.

The Doctor knew him as a friend, though he did not know of the connection between Salyavin and Chronotis.

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.

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.

_{Biography from the TARDIS Data Core article, licensed under CC-BY-SA}