Magnus (Flashback)

In an incarnation which was not his first, a Time Lord who had acquired the nickname of "Magnus" in his youth was a contemporary of the First Doctor on Gallifrey, with whom he had a falling-out as Magnus's thirst for glory drove him to accounts his old friend found unethical. (COMIC: Flashback) Some accounts implied he would go on to become the Master, (PROSE: Goth Opera) notably highlighting his frivolous waste of regenerations, (COMIC: Flashback, PROSE: Invasion of the Cat-People) but another account claimed that the War Chief was the later regeneration of a "Magnus" distinct from the Master. (PROSE: Divided Loyalties)

History
Magnus was not in his first incarnation and was quite superior about it, mocking his old friend the First Doctor for "still hobbling about in [his] old body". He had obtained the nickname of "Magnus" in his youth, much as his friend had been nicknamed "Thete", and expressed some surprise at how long the nicknames had stuck;

At some point prior to the First Doctor leaving Gallifrey, he and Magnus were good friends. However, one day, Magnus attempted to drain power from a conglomerate of Artron energy retrieved from the Time Vortex. The Doctor realised the "ball of energy" was actually a sentient energy being and stopped Magnus; while the Doctor was commended for his actions, Magnus, who wished for the experiment to bring him fame equal to Omega and Rassilon, never forgave the Doctor for ruining his plans that day, becoming a bitter enemy of his. (COMIC: Flashback)

Ruath later cited "that idiot, Magnus" alongside the Doctor, the Rani, and Mortimus as the group of students under Borusa who had become "scoundrels"; (PROSE: Goth Opera) with Mortimus being the Monk, (PROSE: No Future) this list placed the Master, notoriously the Doctor's schoolmate, as the Renegade formerly known as Magnus. (TV: Terror of the Autons, PROSE: Goth Opera) Indeed, the Second Doctor would later recall advising "Magnus" to be more careful with his regenerations, (PROSE: Invasion of the Cat-People) while the Fourth Doctor highlighted the fact that the Master had gone through his regeneration cycle at an accelerated rate. (PROSE: Doctor Who and the Deadly Assassin)

However, a later account claimed that the "Magnus" who attended the Academy with the Doctor was a distinct Time Lord from "Koschei", the one who became the Master. According to this account, it was also this Magnus who later regenerated into the War Chief, (PROSE: Divided Loyalties) contradicting other accounts' contention of the War Chief as another incarnation of the Master. (PROSE: Doctor Who and the Terror of the Autons, Doctor Who and the Doomsday Weapon)

Behind the scenes
Magnus' nature is a peculiar one; he was intended to be a younger version of the Master by the authors of Flashback, and the story read in this spirit by many. However, when Gary Russell was asked about the names of the Deca, he explained that this conflicted with Dave McIntee's notion in The Dark Path that the Master's original name was "Koschei" instead. For this reason, in Divided Loyalties, Russell decided to retcon the name of Magnus as being that of the distinct Time Lord who would also become the War Chief, as distinct from the Master. An earlier draft of The Dark Path had also had Koschei informing the Doctor that their mutual friend Magnus had fled with a supply of spare TARDIS parts, identifying Magnus as the War Chief; McIntee later published the scene in the charity anthology Perfect Timing.