Magnus (Flashback)

In an incarnation which was not his first, a Time Lord known as Magnus 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, highlights 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".

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)

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.