The Heretic was a Time Lord who believed that the universe was sick after staying on Gallifrey and seeing the information the Time Lords kept bringing in - he saw wars, diseases, cosmic disasters. He planned to regenerate the universe and start fresh, his Time Lord background leading him to conclude that regeneration was the only way to cure the universe of this illness.
He created the Cult of the Heretic from individuals of many races to help him. They constructed plans for the Anomaly Cage, which would allow the Heretic to activate the universe's renewal using Time Lord biodata, although it would still require a suitable catalyst to destroy the universe. The Heretic was stopped by the Doctor before his plans could be carried out. The Heretic was either killed by the Time Lords or sent to Shada. (AUDIO: The Two Masters)