The Heretics are an antagonistic group within Ellie Troits Bonding Arc. They have no part in the main story.
The Heretics are a group that stole forbidden artifacts and plans to rebel against the Attera Church to create a new world order. The Gracis Cardinal Cabinet is trying to eliminate them. They are the sworn enemies of the Stillbloods. They are a splinter group from the Aterra Church. Ten years ago, some priests ran off with brainwashing spells. These same priests now seek to create a new world order. Though it has been stated that they seek to create a world founded on their beliefs, these beliefs are never mentioned; Their ideology, religion, or any form of motivation or a belief system is still a complete mystery. The fact that their sole goal has been to gain followers through brainwashing, then it's reasonable to assume they wanted complete power.
Barriers are invisible magic walls that the Heretics use as bases of operations for brainwashing. The only known way to destroy them or even find them is with an X-Machina, a special weapon distributed by the Attera Church to Stillbloods with special properties that penetrate the barriers through the use of the Stillbloods' massive Star Energy deposit. The barriers are highly conducive to performing brainwashing spells, due to the fact that barriers aren't easily broken. The Heretics attempt to brainwash large factions of people, but the process could take months. The barrier of an active brainwashing facility is extremely strong, even for the God's Gift, who was badly affected. That's why Stillbloods are used to fight them there unique physiology, which makes them very resistant to mind manipulation. Though the X-Machina can usually detect barriers, the Original [Priests] demonstrate the ability to use dark crystals to camouflage themselves to avoid being detected.
The Originals are the original ten priest that broke away from the Attera Church with the the Forbidden Spell. The Originals are the group that ate the heart of the rebellion, though the details are unknown. The ten priests are many times stronger than an average Heretic. Lucian self-proclaimed that he could easily take one or two Heretics, but he would need at least five Stillbloods with him to fight one Original.