Magnus (Flashback)

Magnus was a Time Lord contemporary of the First Doctor on Gallifrey. (COMIC: Flashback)

History
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 realized the "ball of energy" was in fact a sentient energy being and stopped Magnus; while the Doctor was commended for his actions, Magnus never forgave the Doctor for ruining his plans that day, becoming a bitter enemy of his. (COMIC: Flashback)

According to one source, Magnus went on to become the renegade Time Lord known as "the War Chief" to the War Lords. (COMIC: Timewyrm: Revelation)

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 had the name of Magnus used for the War Chief in Revelation. 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.