The Arcane Assembly was a parliament of mages in Shornhelm, acting for most of the area's history as it's de jure and de facto government. However, since the installation of the Lariat family as the first Kings of Shornhelm by the Elder Council in the Third Era, its power has greatly diminished. The Arcane Assembly has not met in the last century, and is generally taken to be defunct.
The Arcane Assembly began during the Direnni Hegemony as a bi-annual conference of mages in the Shornhelm, where the local magelords would meet in the capital for the purpose of magical debate. However, the original magelords were as loathe to share the painstaking research they had conducted in their Ivory Towers with rival mages, the Shornish an insular, distrustful race of man and mer at the best of times. As a result of this, the original Arcane Assemblies quickly transformed into opportunities to petition the Direnni government for matters that effected the magelords of the northern Wrothgarians as a whole, and they would continue in much this vein. Consisting of every single magelord in the Shornhelm area, a part of High Rock traditionally neglected by the Direnni ever since its first founding, the Arcane Assembly became the closest thing to a government the area had ever seen without any fuss or fan-flare. Whenever any Shornhelm-wide decision was to be made, a vote of magelords present would be held.
The Fall of the Direnni Hegemony had little to no effect on Shornhelm: as an unwanted, snowy mountain range, it has, not surprisingly, held little interest to conquerors throughout its history. The Arcane Assembly continued to meet, even through the Second and Third Empires, much the same, called only whenever some issue that troubled the entire area occurred (most normally an issue with a Daedra Prince, or the traditional raids northwards to gather slaves/test subjects from the Northpointers).
The most famous meeting of the Arcane Assembly was perhaps in 3E 266, when they agreed to throw in their lot with the Camaron Usurper, Haymon Hart-King, and his undead horde which was invading High Rock at the time. Seeing an agreeable amount of leverage to be gained with the Princes of Oblivion and an invaluable wealth of access to examples of necromancy at it's finest, the Usurper's war was also a handy excuse to gather the magelords together and make a rare raid southwards. Following the Usurper's legendary defeat at the hands of the alliance of Ykalon, Phrygias, Kambria and Dwynnen, Shornhelm's reputation was somewhat diminished in High Rock, and the area suffered many retribution wars in the following years, the Assembly painted as one of the famous "Eight Traitors".
Using this shameful act as leverage, King Allard I, the son of Andorak Lariat, began the process of stripping the Arcane Assembly of it's once limitless powers. The Lariat family had been installed as The Kings of Shornhelm by the Elder Council shortly before, though they had failed to exercise any control on most of their new dominion, due to the blind loyalty all peasants of Shornhelm held towards their local magelord, and this the Arcane Assembly. Following the Fall of the Usurper, Allard Lariat began a long series of reforms that made it more and more difficult for the Assembly to ever meet, and thus ever effectively combat his royal influence.
Throughout the next century, the Arcane Assembly effectively ceased to exist, with no nation-wide magical policy existing. Following the conquest of Shornhelm by Northpoint in 4E 25, The Arcane Assembly was officially dissolved, and removed from Shornish law.