Magnus (Flashback)

"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". (COMIC: Flashback)

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)

According to a nightmare the Fifth Doctor had under the control of the Celestial Toymaker, Magnus went on to become the renegade Time Lord known as "the War Chief" to the War Lords. (PROSE: Divided Loyalties)

The Eighth Doctor mentioned Magnus when reminiscing about the Deca, listing him alongside Koschei as a childhood friend. (AUDIO: Neverland)

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 an alternate young version of the Master going by "Koschei", created by Dave McIntee. 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.