The War Chief

Magnus (a.k.a. the War Chief), was a former friend of the Doctor and later, a renegade Time Lord who assisted a group of aliens known as the War Lords.

Life on Gallifrey
Magnus in those days belonged to the same clique as the Doctor, who has youths called themselves the Deca. (PDA: Divided Loyalties).

A holo-simulation shown by the Doctor to Bernice Summerfield detailed the later days of their relationship. According to the simulation, Magnus grew into an ambitious and arrogant Time Lord who already had little time for "Theet". Magnus engaged himself in a scheme to provide Gallifrey with a new power source derived from a sphere of artron energy and, in so doing, re-vitalize the decadent Gallifreyan culture. The Doctor made himself an unwelcome guest observer of the experiment. He realised the sphere was in fact a living being and that Magnus' actions were killing it. The Doctor sabotaged Magnus' equipment and freed the creature, an act for which the Time Lords commended him. This angered Magnus even more. He and Magnus were bitter enemies from that day on. (DWM: Flashback)

Ally of the War Lords
Now calling himself the War Chief, he assisted the War Lords, who had abducted soldiers from wars spread across Earth's history to play in simulated versions of the wars from which they originated. Thinking Humans the most vicious species in the galaxy, the aliens hoped to later pit the survivors against each other and to use the humans to conquer Mutter's Spiral.

The War Chief aided the War Lords by giving them techonlogy to build basic TARDIS-like space-time machines, SIDRATs, which they used to kidnap the human soldiers and travel between era-specific zones which they had created. The War Chief and the Doctor met and recognised each other. The War Chief solicited the Doctor's help to double-cross the War Lords and seize power for themselves. The Doctor pretended to accept the War Chief's offer.

At the same time, the Security Chief of the operation distrusted the War Chief, believing he meant to call in the Time Lords. The two engaged in a series of machinations against each other which ended with the War Chief disgraced, though he shot his rival dead. Unable to resolve matters and return the soldiers to their own times, the Doctor summoned the Time Lords for aid, while the War Lords discovered the War Chief's plans and executed him, or so they believed (DW: The War Games) Unknown at the time, the War Chief did not die but, rather, underwent a faulty regeneration. His new form appeared like two bodies fused together. He took to wearing cloaks and hoods to disguise the fact. (NA: Timewyrm: Exodus)

Aftermath
Hiding his misshapen doubled body in cloaks and hoods, the War Chief served as an occult advisor to Adolf Hitler in hopes of changing history. The Doctor confronted him once more at this time. (NA: Timewyrm: Exodus)

Personality
Behind the War Chief's actions lay real idealism, tainted with powerlust.

Behind the Scenes
FASA's The Doctor Who Role Playing Game identified the War Chief, the Monk and the Master as different incarnations of the same renegade Time Lord. Comics and original novels have since contradicted this. The non-canonical fan novel Time's Champion identifes the War Chief as a previous incarnation of the Master.