Complex sentence: The Master is the creator and leader of the super mutants and is also the mastermind behind a movement called "The Unity", the aim of which is to turn every human possible into a super mutant by means of the FEV virus.
Split into simpler sentences: The Master is the creator and leader of the super mutants. He is also being the mastermind behind a movement called "The Unity", the aim of which is to turn every human possible into a super mutant by means of the FEV virus.