The War Chief

The War Chief, known at the Time Lord Academy as Magnus, was a former friend of the Doctor and, later, a renegade Time Lord who assisted the War Lords. After the failure of the War Lords, he regenerated, although he was deformed by the experience. He then travelled back in time, in order to use the Nazis as his agents. He was finally stopped by the Seventh Doctor.

Life on Gallifrey
Once called Magnus, he belonged to the same Academy clique as the First Doctor, the Deca. (PDA: Divided Loyalties).

The Doctor once advised Magnus to reserve his regnaration, but Magnus did not listen as he was not concerned. (Invasion of the Cat-People)

In the later days of their relationship, Magnus became an ambitious and arrogant Time Lord with little time for "Theta Sigma". Magnus engaged in a scheme to provide Gallifrey with a new power source derived from a sphere of artron energy, hoping to re-vitalise the decadent Gallifreyan society. The Doctor made himself an unwelcome observer of the experiment. He realised the sphere was in fact a living being and that Magnus' actions were killing it. He sabotaged Magnus' equipment and freed the creature, for which the Time Lords commended him. This angered Magnus even more. The Doctor 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. They abducted soldiers from wars spread across Earth's history to play in simulated versions of the wars from which they came. Thinking humans the most vicious species in the galaxy, the aliens hoped to pit the survivors against each other and to use them to conquer Mutter's Spiral. The War Chief aided the War Lords by helping them build SIDRATs, TARDIS-like space-time machines. They used them to kidnap the human soldiers and travel between era-specific zones which they had created. The War Chief and the Second 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. (DW: The War Games)

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. 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. (DW: The War Games) Unknown at the time, the War Chief did not die but, rather, underwent a faulty regeneration. His new form looked like two bodies fused together. He took to wearing cloaks and hoods to disguise the fact. (NA: Timewyrm: Exodus)

Aftermath
The War Chief served as an occult advisor to Adolf Hitler under the name "Doktor Felix Kriegslieter", hoping to change history with the Nazis as his agents. The Seventh Doctor again confronted him, prompting the War Chief to try to take possession of the Doctor's healthy body and his remaining six regenerations. However, his efforts to replace Hitler with Heinrich Himmler were thwarted by Himmler's devotion to his Führer (as opposed to his other officers, who served Hitler out of necessity), allowing the Doctor to alert Hermann Goering to "Kriegslieter's" betrayal and destroy the War Chief's base by overloading its nuclear reactors. His agents were killed by the Nazis due to their lack of initiative. (NA: Timewyrm: Exodus)

Personality
The War Chief was an ambitious and arrogant individual, cunning, and with great tactical abilities. He pretended to loyally serve the War Lords, while plotting to take control of them. He was unconcerned with using up regenerations, and never listened to the Doctor, who advised him not to waste them. Behind the War Chief's actions lay real idealism, tainted with power lust. (MA: Invasion of the Cat-People)

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. However in the module Legions of Death he appears as a Renegade Time Lord who is a former ally of The Master. In addition, the non-canonical novel Time's Champion indicates that the War Chief is in fact an early incarnation of the Master. Canon, comics and original novels have since contradicted this.
 * The necklace prop the War Chief wears in The War Games is the same prop previously worn by Zephon in The Daleks' Master Plan.