The Magisterium was an magical organisation that regulated the education and usage of magic as well the governing of The Empire. In its first incarnation it only acted as a regulator of magic and quickly declined shortly after, it's revival during The Interregnum was its real rise to power and transition to a political body.
As a regulator of MagicEdit
Shortly after the end of the (UNDEAD INVASION, ETC) the Magisterium was created by a cabal of Cannor's most prominent mages to make sure no large-scale magical disaster ever happened again - the Magisterium shortly enacted the Maielan Laws which included the ban of black magic and the standardization and reform of the magical syllabus taught at the arcane colleges at the time.
As a governing bodyEdit
The Magisterium's formation of The Empire saw the end of the bloody period known as the The Interregnum and the Magisterium became a governing body that policed entire kingdoms and nations, with a Grand Magister being elected by from the Magisterium, by the Magisterium.
After the Wars of Rule the absolute power of the Magisterium waned and they no longer fully controlled The Empire. The Concordat of Rule turned the Magisterium once again into a a regulating and advisory body (rather than ruling), with the Grand Magister being chosen as the spiritual and magical authority of The Empire and an actual Emperor (elected by The Magisterium and other Electors) from the various magical nobility of The Empire served as the physical and political authority of The Empire.
Magister: the standard rank, only the upper percentile of magical academy students are permitted entrance in The Magisterium, with only magical talent taken into consideration, not race or social rank. Most Magisters are assigned as advisors to lords across The Empire, performing the roles of a court mage, consultant, and of course, the eyes and ears of The Magisterium. Magisters who are given special tasks that require them to be more mobile are unofficially called 'Free Magisters'.
High Magister: High Magisters are given a region oversee and all Magisters in that region report to them. High Magisters do not reside in the court of nobles but have their own residence or towers that act as a base of operations for the Magisters of that region. Each High Magister is also part of the High Council which forms the upper ruling body of The Magisterium, the High Council decides the plans of The Magisterium, who becomes a Magister, and most importantly, the election of the Grand Magister. The most powerful High Magisters are also Electors and have a hand in choosing the next Emperor.
Grand Magister: in it's early incarnation the Grand Magister was equivalent to the Emperor, now the Grand Magister serves as the head of The Magisterium and the spiritual and magical authority of The Empire as a whole. The Grand Magister is chosen from the ranks of the High Council and each Emperor-candidate must be approved by the Grand Magister before they officially take on the role.