Time Lord messenger (Terror of the Autons)

A Time Lord messenger came to Earth to warn the newly exiled Third Doctor about events. (TV: Terror of the Autons)

Origins
In his previous incarnation, he was a member of the tribunal who sentenced the Second Doctor to exile on Earth (PROSE: Doctor Who and the Terror of the Autons)

Encounters with Third Doctor
Being on the High Council, the messenger was sent on orders of Lord President Pandad IV as a direct result of Rassilon's agent Socra being asked by his Matrix Lord masters to keep an eye on the Doctor. (PROSE: The Legacy of Gallifrey)

He first encountered the Third Doctor after his regeneration when he appeared in his TARDIS while the Doctor was with his companion Liz Shaw. (AUDIO: Shadow of the Past)

After the Doctor's defeat of Helios, he spoke to the wardens of Shada, requesting that they release and keep the Doctor busy on Earth. (PROSE: Prisoners of the Sun)

He then travelled to Earth to warn the Doctor of the Master's arrival and of an impending trap that the Master had set for the Doctor. The Doctor observed sarcasm was not his strong point. When the Doctor asked for assistance in dealing with the trap, however, the messenger simply disappeared. (TV: Terror of the Autons, PROSE: Doctor Who and the Terror of the Autons)

He was sent to see the Doctor again when he was with Sarah Jane Smith in order to persuade the Doctor not to travel with a future companion whom the Doctor had not yet taken on board the TARDIS. (PROSE: An Overture Too Early)

Later activity
The messenger told the Sixth Doctor about the many different versions of Peri Brown that were in the universe after the High Councils interfered with events of Thoros-Beta. (AUDIO: Peri and the Piscon Paradox)

He also once met the Eighth Doctor and Charlotte Pollard. (AUDIO: The Man Who Wasn't There)

Behind the scenes

 * Although Terrance Dicks' novelisation of Terror of the Autons revealed that the messenger had been part of the tribunal that sentenced the Doctor in The War Games, it was unclear which member of the tribunal the messenger regenerated from. The Legacy of Gallifrey by Gary Russell names the three members of the tribunal as Goth, Socra, and Adelphi, while also featuring the messenger as an unnamed member of the High Council. Because it explicitly establishes Socra as separate from the Time Lord messenger (and implicitly Goth), The Legacy of Gallifrey could be understood to connect with Doctor Who and the Terror of the Autons if one assumes (contra to Russell's syntax) that Adelphi and the messenger are the same individual. However, Russell's novel Legacy shows the entire tribunal (including Adelphi) still in their  War Games incarnations at a later point in the timeline.