Heretics are a blanket term used by The Order to describe any sort of group that doesn't agree with or actively subverts the wills of the gods. They can be tribals that refuse to listen to the words of the gods, Robin Hood-esque gangs that don't agree with the gods, people who believe in humanity above all, or murderous bandit bands that use heresy as a way to further their own gains or justify their actions. They're everywhere across the Continent, and despite the Order's best attempts to wipe them out, they simply pop back up again if wiped out.
Heresy and heretics go back to the first days of humanity, and the first days of when the Ancient Gods contacted humanity. As it is with all things, along with those who accepted the words of gods, there were those that didn't, and those that didn't were soon branded heretics. They ended up being a social minority, with people more willing to listen to the voice from the sky telling them what to do and how to do it then the naysayers on the ground. Along with the advice of the gods, heretics were mostly driven out of the limelight or forced to hide their beliefs.
With the formation of further civilization and city-states came the churches, organizational bodies dedicated to carrying out the will of the gods. Along with them persisted the heretics, in hiding or openly carrying their defiance with their words. When the churches organized and began to wield unprecedented political power, they cracked down on heresy, wiping out heretical city-states or rulers in a series of crusades and wars that lasted all throughout the Wild Era. When the Alammar Empire rose and conquered the world with the help of the gods, heretics were one of the first on the chopping block. The Fervoni Holy Union, and by extension the gods, demanded that they all be wiped out, and William the First agreed.
The idea of heresy was not wiped out in those millennia. Heretics kept their ideas hidden and secret, and they passed their resistance on to their children, and their children onto their children. Some were turned to the idea by injustices done by the church, others simply to spite the Empire and the world. Regardless, the idea of disobedience lived on, if not in those original bloodlines, then in the new disbelievers turned by circumstances or word of mouth. They hid in the shadows and kept their secret in the closet, and waited, for something they hoped would come.
When The Splintering came and the Empire collapsed, followed by a Period of Chaos, the hidden heretics were overjoyed. There was no authority keeping their beliefs down and the Fervoni Holy Union was in no shape to crack down on whoever didn't believe in their sayings. Heresy flourished, the idea spreading across the Continent like a plague.
Eventually, this too would come to pass. When order was inevitably restored, and the Order having gathered enough strength by itself, heresy was once again pushed to the edge of civilization, where it lies to this day. They lie scattered in the warring ruins of Ravenna, in the jungle of The Yunnaiteddo Federation, on the shores of Falglor, at the edges of Valdar, hidden in the cities of Adhyria and far from the reach of the Order in the Eari Isles.