Master

The Master, was the brain behind the Mutant Army. He was born in Vault City as Richard Moreau, where he was a doctor, but was exiled from there for murder, which he presumably did not commit. He changed his name to Richard Grey and went south to a town called Hub. There he became a leader of a team of scientists who mounted an expedition to an apparently abandoned Military Base to the north. Among them was Harold, who became a good friend of Richard Grey during the course of the expedition.

The Military Base still had active automated defences that killed a few of the expedition's members, but they continued exploring deeper into the base. On the its lowest level, they found the Forced Evolutionary Virus vats. Richard Grey was knocked into one of the FEV vats by a crane. Harold and the surviving members of the expedition thought Richard was killed and decided to escape the base to avoid his fate. Most of the expedition's members were killed on their way out of the base by sentry robots and other defensive mechanisms. It seems that only Harold was able to get back to the Hub.

But Richard had not died. He was one of the very few "lucky" humans to be modified by the FEV without a reduction of his intelligence. It was increased, in fact. Richard's mind and body were supposedly melded with many other personalities, his diaries seem to indicate, and shortly he felt stronger and more capable than before. With this new confidence he decided to start unifying the wasteland to make it as perfect as he had become. He begun his search for humans to dip into the FEV to create his mutant army. Unfortunately, the results were not as great as with him: most often, the dipped just mutated into dumb big soldiers known as Super Mutants. Only those with low total radiation count did not suffer this fate.

Related Holodisks
Richard Grey Audio Diary

The Master in Fallout games
The Master appeared only in the first Fallout game. He was mentioned in all other games, though, and his being previously called Moreau and being exiled from Vault City was revealed in Fallout 2.