Super Mutant

Super mutants are mutated humans, product of Forced Evolutionary Virus infection. They have (mostly) green skin, are immune to disease and radiation, and blessed with exceptional strength and endurance. They also possess the exceptionally long life of the mutated as well as total sterility.

Super mutants were created by Richard Grey, who led the expedition to Mariposa Military Base. When the expedition was attacked by robots, he was himself dipped in a FEV vat, and managed to crawl out. FEV was initially meant to be injected into humans, but Grey soon found out that direct physical contact worked as well. He began to mutate in horrible ways, turning into a sort of blobby kind of thing. He developed psychic powers, which were enhanced by consuming living minds to expand his own brainpower. Slowly, as wanderers made it into the base themselves, he started doing his own experiments with FEV. Grey--who now called himself the Master--lamented the needless destruction of the Great War. In his warped state, he decided that he would have to force humanity to evolve. If everyone could be as perfect as his super mutants, there would be no more conflict. Those who could not evolve would die. The Master began seeking out uncontaminated humans with which to create mutants and build his super mutant army.

However, creating super mutants was a very hit or miss process. The great majority of super mutants produced by the Master and later his Lieutenant in Mariposa's Vats were big, dumb brutes. Physically, there were vastly superior to humans, but they had the intelligence of children. What exactly causes some mutants to be brilliant and others to be stupid is unknown. The Master was certain it was related to radiation damage: humans who hadn't been exposed to much radiation yielded better mutants (see his personal diary for his thoughts on this). His Lieutenant, however, had a different theory. When the bombs hit the research facility and turned it into the Glow, they cracked open a few tanks filled with FEV. The bombs' radiation then mutated this FEV into an airborne strain. But this new airborne FEV didn't have any real mutagenic effects on people. All it did was inoculate human subjects against the real FEV, acting as a sort of vaccine. The ideal dipping subject was someone who hadn't been exposed to the airborne FEV. Both these conditions--no radiation exposure and no mutant FEV inoculation--were present in one population: Vault dwellers. Each one contained around 1,000 viable subjects to be dipped and turned into super mutants. Which factor exactly determines what you'll get from a dipping--radiation or inoculation--is still uncertain, though as a rule of thumb, the cleaner the subject, the better.

Appearances in games
Super mutants appear in all Fallout games.