The Black Circle is a group of republic mages who in principle, own the Well. They were formed back in the days of the Empire when they were in service of the Emperor. They have survived to this day, and are now in service of the Primearch and his Archons.
What do they do?The Black Circle is a group of founts who take on many fount-related missions. They do research, gather knowledge, spy, and sometimes train Honorguard and Seppies in using fount. To most Republic citizens, they are a mysterious order of which they known nothing about. When a Black Circle Priests or mage or whatever they call themselves these days, walks around the city, people will get out of their way or even scatter completely. The group has been known to influence plenty of historical events. They worked directly with the Malen to defeat the corruption for example. They were also there when a man got himself the Necklace of Kasssrak and blew his village up with it. They never got out of him where he'd hid the necklace though, unleashing all sorts of chaos on the world. Until this day, the necklace is still unaccounted for. Not that there haven't been searches, but like most important things, the necklace seemed to vanish when needed most.