Officially, the Church of Vaarta abhors magic and its practitioners as witches and heathens. Thus it is written in the Book of Iantash.
However, the Ulvoski (meaning "Painted Men") are a branch of the church composed of magic-wielders. Made official by the Accords of Murmask, the Ulvoski Order are said to derive their powers from Markam himself.
Dark Sun has the Sorcerer Kings jealously guarding their magical superiority by having their Templars root out other magicians, especially Preservers who have joined to form the Veiled Alliance.
www.athas.org has 3.x edition stuff, I think.
Or...
It's only sketchy, but you might want to look at the Khazarate of Vaarta:
http://www.splendour.freeservers.com/vaartamain.html
Set up a pop-up blocker though - Freeservers kept increasing the number of adverts in an effort to extort money out of me. Additional information for Vaarta relevant to your enquiry that was newer than the site (references will be more clear if you read the Vaarta pages first) :
"The Ulvoski are hated and feared by peasant, noble and clergy alike. Clad in hooded black robes, black gloves with even their faces painted black, the Ulvoski are designated as Church inquisitors, their remit to seek out heathen magicians and either convert them or destroy them. Male magicians who submit are indoctrinated into the Ulvoski through hard and painful trials of spirit and flesh. Those who refuse are given the Five Wounds of Markam to purify them, and then killed. Women who use magic are always killed in this fashion as witches.
Over time, the powers of the Ulvoski have grown so that they no longer seek out just magicians, but heretics of any kind. The leader of the Ulvoski Order, Borstin, is at odds with the current Patriarch in a bitter yet secretive power struggle. Borstin would like nothing more than to see the Ulvoski running the entire Church. He flaunts his extreme asceticism and piety in the face of the luxury of the Medinavic clergy, yet secretly he has searched through forbidden books and undertaken dark rituals. In his pride he hopes to conquer Kebel with his power - an aim that can only end in disaster one way or another. "
The irony here is that the Ulvoski, in order to better root out magicians and heretics, are themselves magicians...