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Demonmasters are very rare, and for good reason. Nature did not intend for magi to be able to summon Demons.
They often only have one or two demons that they're truly the master or mistress of, and completely control (if any at all; weaker demonmasters are often killed the first time they attempt to summon their familiar because they aren't strong enough to control it), but that one demon will eventually grow incredibly close to and protective of its master or mistress, despite originally being more aggressive toward him or her.
Other magi will not take a demonmaster as an apprentice until he or she has proven that they can control their familiar.