Brotherhood of Sorcerers

The Brotherhood of Sorcerers was the oldest organization of mages in the Northern Kingdoms. Formed in the aftermath of the non-aggression pact established by the Novigradian Union in the 8th century, it united and had control over most of the magic users, magic academies like Aretuza or Ban Ard and institutions, such as Rissberg. It was It was led by the Conclave, which consisted of two main governing bodies that guided magical practice for centuries before dissolving completely as a consequence of the events that transpired during the Thanedd coup in 1267.

Conclave of Mages
The most distinguished practitioners of magical art first founded the Conclave of Mages to institute codified rules on the use of magic, but the creation of a new hierarchy of order sparked brief civil war, costing the life of Raffard the White, who opposed its formation. Later, another body, the Supreme Council of Sorcerers, was established to aid the Conclave in administering and regulating practitioners of magic. In particular, the Council focused on magical experiments, research, and expanding magical knowledge. Within the Conclave, the Chapter of Mages formed the higher chamber, while Council of Mages acted as the lower chamber, but the latter also wielded important authority in the Brotherhood.

After the coup, former Council member Philippa Eilhart established the Lodge of Sorceresses. Unlike the Brotherhood, the Lodge is female-only and includes sorceresses from both the Northern Kingdoms and the Nilfgaardian Empire.

Reading the book "The Conclave of Mages" provides a glossary entry:

Glossary entry

 * The Conclave was, next to the Supreme Council of sorcerers, one of the two main bodies ruling the magicians. The most powerful sorcerers of their times sat on it. One of it's duties was regulating the standards and rules of using magic - the ban on necromancy was one of it's edicts. Before the Thanedd rebellion, a large part of the Conclave conspired with Nilfgaard. During those events most of it's members died, the sole survivor being Francesca Findabair. After the coup the Conclave was not reactivated, thus many mages do not respect the restrictions it had created.

Prohibitions
Throughout its existence, the Conclave made a number of restrictions regarding certain branches of the magic, like necromancy, goetia or experiments on genotypes of the intelligent races. Individuals who ignored the ban, used to be excluded from the Brotherhood and were considered as renegades, e.g. Alzur or Idarran of Ulivo. However, though all mages are equal to the law, same of them are actually more equal than others; an open secret was common breaking the bans in Rissberg until it came to light after deaths of Ortolan and Sorel Degerlund.