The charred folk were exiled by the more tolerant members of the Church of Zamorak for their violence, presumably during the time when both Saradominists and Zamorakians worked together before the burning of The Wizards' Tower. They followed the teachings of The Chaos Priest Lungrim, which were thrown out of the Church by its aforementioned more tolerant members.
Sulla was chosen by his father to be their next leader. Shortly after his birth, his mother was slain, and Sulla baptised in her blood. Years later, he swore loyalty to Zamorak as the elders lowered a red hot metal mask onto his face, and that he would carry the banner of Zamorak to all the corners of the world.
After he was scarred, he swore revenge slaying his father and the priest who carried out the ritual, and burning their place of worship to the ground. He returned many years later with an army of Kinshra Soldiers and enslaved the remainder of his people to work in the Kinshra mines, where the last of the charred folk presumably died.