The Brotherhood of Magisters are an ancient sect of magic users founded during the Dark Ages in which they thrived as a society. With their power diminished in modern times, the Magisters desired to return the world into its archaic state so they could rule it.
The Magisters ruled during the Dark Ages, when their magic inspired fear. However, as technology advanced and modern times took over, their power declined and their times faded away. They have roots in many places, including the police, as an English policeman is seen being a member of the Magisters. They also dress and speak in the style of ancient times; when in public, they will wear modern clothes. Their primary weapon is a spell they use to fire electrical discharges from their hands.
In the 21st century, the Magisters stole the ancient Stonehenge, believing it to be a magical weapon of mass destruction (while others say it calls aliens) by simply lowering it below ground. By happenstance, Jackie and Jade Chan came upon the Magisters by accident, as Jackie was doing an archaeological dig in the very castle the Magisters used for a fortress, and accidentally fell down a trap door. The Magisters believed that Jackie heard their plans, and began to chase him all over London before the Chans decide to investigate.
The Magisters eventually captured them and decided that, since they know too much, they revealed their plan to use the power of Stonehenge to destroy every major city and plunge the world into a new Dark Age for the Magisters to rule once more. But over the phone, Uncle used a spell that cancel their magic. With their spell weakening, the head Magister used one last blast, and Stonehenge fired into the sky, only for London to not be destroyed. Interpol agents then came in to arrest the Magisters. With Jackie's testimony, the Magisters would be locked up for a long time. But, unknown to everyone else, Stonehenge did do something: it left a signal for a flying saucer.
The magisters may in fact be based on real world practices such as Hermeticism and organizations such as The Golden Dawn or Freemasons.