"Magnus" was the nickname of a Time Lord contemporary of the First Doctor on Gallifrey. (COMIC: Flashback)
History
The Doctor once advised Magnus to reserve his regenerations, but Magnus did not listen as he was not concerned. (PROSE: Invasion of the Cat-People) He even mocked the 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)
The Fifth Doctor would later experience nightmares of his time at the Time Lord Academy in the Deca, where the nicknames "Magnus" and "Koschei" were used by two of the other members of the group. According to this account, the Magnus the Doctor had dreamt about went on to become the renegade Time Lord known as "the War Chief" to the War Lords while "Koschei", corrupted by the power of the Darkheart, became the Master. (PROSE: Divided Loyalties) Notably, however, yet another account, when relating Koschei's corruption into the Master in the Darkheart incident, alleged that he had at the time only recently started going by the name of "Koschei", with which the Second Doctor was unfamiliar. (PROSE: The Dark Path)
Ruath described "that idiot Magnus" as one of four notable renegades, alongside the Doctor, Mortimus, and the Rani, who had studied together under Borusa. (PROSE: Goth Opera)
When the Seventh Doctor broke into the Monk's TARDIS, the Monk initially assumed it was Magnus come to steal the Monk's gold. (PROSE: No Future)
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.[1] However, when Gary Russell was asked about the names of the Deca, he explained that this conflicted with an alternate young version of the Master going by "Koschei", created by Dave McIntee.[2] In an attempt to resolve the contradiction, Russell thus decided to retcon Magnus as being instead a young War Chief, and thus used the name of Magnus for the War Chief in Divided Loyalties. Strictly speaking, neither identity of the original Magnus has been confirmed; either way, it is worth remembering that the Master and War Chief are sometimes thought to be one and the same, to begin with.
In a deleted line from the AUDIO: Neverland script, the Eighth Doctor mentions both Magnus and a certain "Koschei" when reminiscing about the Deca.