feudal covens

Most human nations in Scarterra use a variation of the feudal system for government. Most feudal lords want to have powerful arcane spell casters that are loyal to them.
One way that some choose to obtain this is to establish feudal covens.
To establish a feudal coven, a king (or less commonly a duke or a count) will bestow a piece of land to a coven of wizards. The coven is allowed to oversee the day-to-day activities of that piece of land and rule over the peasantry there deriving income from the crops, timber, minerals, fish, or reagents produced on that land.
Traditional feudal rulers owe their lieges military services, money, or both. Feudal covens owe their liege magical services instead of or in addition to the usual payment to one's lord.
Feudal covens are usually free to select their leader via any means they want. Usually covens are meritocratic giving the top leadership spot to the wizard with the longest tenure or the winner of a magical duel or contest.
Like most feudal contracts, feudal covens are generally meant to operate in perpetuity with the coven continually answering to their lord's heir after their liege dies and the coven electing a new leader every time the coven leader dies.

Execution

Generally covens tend to have a large portion of land, on par with most duchies. Ideally, the land given to a coven has a magic font or is otherwise rich in reagents.
It is rare but by no means unheard of for a coven to control a fiefdom as a purely autonomous fiefdom.  This usually occurs when an unusually powerful coven manages to claim a magic font and the land around in a relatively isolated area.

Participants

Most wizards in covens usually find the day-to-day running of their realms as annoying burden.
Some of them pass the managerial aspects of their realm off to apprentices, but most will hire a bailiff to handle the mundane aspects of running a feudal realm.