Magical Law is a system of customary regulations on Equus. Its status in relation to systems of civil and criminal law, and the degree of its endorsement by sovereign states, has differed considerably over the thousands of years of its history.
History
Distinction between Magical and Civil law first emerged in the Unicorn Kingdom in the late fourth century BE. A royal stele attributes the title "Hereditary Guardian of All Legitimate Magic" to Fancy Crown, the crown prince of the Unicorn Kingdom, in 298 BE. The codification of customary magical law on which the current system is based, the Canons of Starswirl. The Canons date from the unification of the Unicorn Kingdom and Equestria, and are likely pseudonymous, Starswirl the Bearded having died in 21 BE - though their attribution to an apprentice, Clover the Clever, has supporters among historians of magic.
The development of Magical Law as an entity distinct from the mundane legal system is believed to have had a twofold impetus. First, it is generally considered that only mages had the requisite skills to detect and investigate many forms of magical criminality. Secondly, magical crime often crossed jurisdictional lines - the negative results of magical evildoing could be geographically far removed from the perpetrator. It was therefore acknowledged as desirable that magical jurisdictions should not be coterminous with mundane ones, nor beholden to the same set of political pressures. Magic's history as a self-regulating profession probably also played a part in the development of a separation between magical and mundane law. The great personal power of some magic users also made it a practical necessity necessary to depend on the legitimate magical community to enforce the law against one of their own who had turned to evil, and hence the magical community found itself in a good position to promote the separation and their legal autonomy in magical matters. The separation has rarely been entirely complete, however; for example, the court archmages of high feudal nobility were often the magico-legal superiors of the ensconced archmagi within that noble's territory.
Since the separation, Magical Law has often been without a unified legislative body, changing only slowly as precedential consensus adapted to changing times and mores. At other times, some body has been recognized as authoritative by most magistrates. For example, the Equestrian College of Archmagi, convened by Princess Celestia in the early Solar Age after the banishment of Nightmare Moon, reformed the Canons of Starswirl and added a second group of laws, the Celestial Canons. That body met several times over thirty years, but was not called on a regular basis. By the 37th century AE, a permanent legislative body of Magical Law, the Harmonic College of Archmagi, was widely recognized on Equus, though the recognition is not completely universal outside the system of Curacies. (I.e. there are some de facto jurisdictions of lawful magic which are not part of any Curacy, and of these, some do not consider the Harmonic College of Archmagi's laws as binding upon them.)
Since antiquity, the original jurisdictions of magical law have been the Sconces; Curacies are a Harmonic-Age innovation that replaced feudal system in which ducal and royal court archmagi were the superiors of ensconced archmagi in their patron's feudal dominion (more or less).
The current system of magical law upheld by the Curacies is the Second Harmonic Code of Magical Law, written by the Harmonic College of Archmagi and promulgated by the Curatrix, Princess Nova, in 3593.