The Imperial Magistratus is the judicial branch of the Imperium's government. They employ thousands of clerks and magistrates to ensure that the Imperial Law is interpreted and executed in the right way throughout the empire. Nominally they are under the control of the Imperial Senate, and are often used in their schemes.
In the cities and provinces the magistrates have fairly limited power as they have to obey thy myriad rules dictated by Imperial Law. However, in newly conquered regions magistrates are charged with introducing Imperial Law to the local population, so they enjoy great freedom at interpreting and executing the law with little to no oversight. In such cases magistrates often employ several gangs of Crimson Tide soldiers to act as their enforcers. Their roles range from catching bandits to confiscating properties and carrying out executions. Magistrates with such postings have a mixed reputation: for every story of an incorruptible, impartial agent of justice there is another about a greedy magistrate trying to extort the common folk.