The Cult of the Heretic were a religious group dedicated to worshipping the renegade Time Lord known as the Heretic. Their goal was to regenerate the universe to become its new rulers. Although they worshipped a Time Lord, none of the cult members were Gallifreyan.
The cult was started by the Heretic, who used the cult to construct an anomaly cage. The Doctor stopped the cult from using the Heretic's biodata to regenerate the universe and the Heretic was taken by the Time Lords.
Needing another Time Lord to activate the restart the universe, the cult made a deal with the Master. They manipulated him into coming into contact with his past self. They then betrayed the Master by switching the minds of the younger Master and the older Master. The act of having a Time Lord inhabiting the body of his past self led to universe beginning to break down. The cult convinced the younger Master that he had just regenerated and hired the Transhuman Sisters of the Unholy Protocol and the Dragonhunters to kill the older Master.
The Seventh Doctor encountered both of the Masters and helped them get back into the right bodies. The two Masters then returned to the Cult's headquarters and killed all of the members. They planned to use the anomaly cage, but were stopped by the Seventh Doctor- ironically using weapons that the Masters had used earlier and forgotten to reclaim from the Doctor- who made sure that the universe remained practically unchanged by its regeneration, his only change being to save the life of his temporary companion Jemima so that she was never killed by the Master. (AUDIO: The Two Masters)