The Reformists were rivals with the Stoics during the San 'Shyuum Civil War. They wanted to enter the Forerunner Dreadnought and advance their technology for better use, but the Stoics thought that it was heresy to do so.
During the climax of the San 'Shyuum Civil War, around one thousand Reformists commandeered the Forerunner Dreadnought and managed to get off the Prophet homeworld. The Reformists ultimately won the San 'Shyuum Civil War, but it came with a heavy price. The Reformists were left with around one thousand individuals, and were forced to selectively breed to prevent inbreeding.[1]