The Master is arenegade Time Lord who had grown up on Gallifrey and opposed the Doctor many times. He threatened the existence of the universe itself on several occasions. His diabolical madness was the result of a never-ending drumming sound inside his head that was a link retroactively installed by the Time Lords on the last day of theTime War to further their own goals.
Both the Doctor and the High Council of the Time Lords agreed he was actually evil. The Fourth Doctor called the Master the "quintessence of evil," while the Eighth Doctor said he was "pure evil."
Using the alias Harold Saxon, the Master passed himself off as human, engineered his election as BritishPrime Minister in2008, and later usedEarth to create a newGallifrey. He later used theImmortality Gate to create theMaster Race and attempted to free Gallifrey from thetime-lock. He was last seen willingly entering the time lock surrounding Gallifrey, preventing the resurrected Rassilon from destroying the Doctor and all of reality.