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. (MA: Divided Loyalties).

Those still friends, Magnus grew into an ambitious and arrogant Time Lord. The Doctor put an end to a schme of Magnus' which would have sacrified a living being made of artron energy to provide the Time Lords with power and in so doing re-vitalize his decadent culture. The Doctor, thinking it morally wrong for the being to die, stepped in and reported Magnus' plans to the authorities, who praised the Doctor for his actions, which angered Magnus even more. (DWM: Flashback).

Ally of the War Lords
Now calling himself, the War Chief, he assisted near-humans who kidnapped soldiers from various wars in separate time period of 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 helped them the War Lords by giving them the 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 himself. He offered to share the power with the Doctor, who pretended to decide to help the other Time Lord.

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 in the War Chief, disgraced, but still managing to shoot him dead.

Believing he could not resolve events hiimself, the Doctor reluctantly summoned the Time Lords for help. The War Lords found out the War Chief's plans to betray them, and, they thought, executed him. (DW: The War Games)

Unseen by the Doctor or his companions, the War Chief did not die but underwent a faulty regeneration, his new form that of two bodies fused together. (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.