Legislatism is the belief that reality can be shaped by magical laws. This is essentially “power is where men believe it to be”. In particular, magical laws are wide-scale, territory-spanning, background enchantments that can cause alterations to reality when certain conditions are met, and thus these enchantments are applied to law.
These laws are not believed to extend throughout the world, only as far as the territories believed to be under the jurisdiction of the group of lawmakers that are using this belief to make their magical laws. Neighboring territories they don’t believe to have power over, will not have their reality shaped by these laws.
If these laws are posited standalone, they will be very weak, for there would be nothing to distinguish them from anything else people might desire magical laws to do, and hence like all those other ideas they must be very weak.
Generally these laws are posited with some backing. If they are backed by a deity, then these are religious law - magical laws that enforce the deity’s version of reality, and are thus based on the deity’s power in terms of adherents in various territories. If they are backed by a sect, they are sectarian law, and based on the power of the sect to work their spells in general, which is based on the sect’s adherents and enchantments.
These laws generally are posited with some “cost” concept that sets it apart from and above “just any desired law” - some sacrifice that only a sect can muster, and therefore limiting the making of these magical laws to just the sects in control of the various regions of the world. In mana legislatism, this cost is a drain on mana. For other forms of legislatism, this may involve giving up anything that is rare, or believed to have power; or it may involve rituals such as mass prayers.
The laws can directly alter reality, to a limited extent. However, since laws are subject to interpretation, the magical laws this allows are also subject to interpretation - so for example, if people in the territory do not believe that a particular action would constitute a crime, then the magical law overseeing it will not pick up on that crime, because people would believe that the magical law would not pick up on something that isn’t a crime. Hence, legislatism creates a new faith ‘sub-meta-magic-system’ wherein beliefs can work differently than in the rest of the world of Faith.
These magical laws are first written as bills declaring what the effects of the magical law would be (a la spellcontracting) and then empowered, resulting a regional spell/enchantment.