To keep Fortunata safe from destructive magic, we destroy all who would use magic irresponsibly.
A network of Secret Magic Courts exists to stop inept magicians literally blowing everybody up by accident, or causing the authorities and local population to see mages as a threat. The courts vary wildly in quality and impartiality, but are only concerned with keeping the lid on magic for mutual benefit.
The Dark Ring are a rogue Secret Court fanatically devoted to stamping out all forms of non-clerical magic. (Think Spanish Inquisition-level fanaticism.)
They drag mages before their underground tribunals and decide their fate by the flip of a coin: Hals-heart: the mage lives, but is stripped of all magical power. Hals-head . . . execution, instant and brutal.
Rumour being rife and the truth hard to establish, the wider mage community and other Secret Courts tend to dismiss tales of the Dark Ring as "fanciful". However, in those regions near where the Dark Ring is strong, Secret Courts work hard to prevent their members earning the mistrust of the population: worse than the Dark Ring would be the Dark Ring with the support of the mob.
The Dark Ring began as a Secret Court in South East Fortunata. They arrested a miracle working prophet who had wandered out of the Scar. When brought up before the court, the prophet preached and worked miracles, converting everybody to the worship of Tessellation, a minor God of Order, who grants them some miracles - "not magic, honest guv..." - that can give them the edge over magicians.
The members of the Dark Ring are sincere, or at least are to start with. However, in reality, their miracles are powered by stolen powers and sacrifices. The whole organization looks awfully like a mystic pyramid scheme. If so, it seems designed to grant power to somebody mysterious and sinister at the apex.
Typical Adventures: hunting down mystics, searching for magical artifacts/items (to be studied, then preserved or destroyed)
Secret Magic Courts - spies, actively works against
Looters - Dark Ring sometimes hires Looter bands to find or steal magical artifacts