The person primarily responsible for codifying the realm’s laws, and making sure that every magistrate in the Empire is doing their job correctly, the Lord High Magistrate is second only to the Empress in how much legal authority that they wield. Rarely is the Lord High Magistrate called upon to officiate in a courtroom setting, but when they are, it is for a matter of the upmost importance.
The Office of the Judiciary routinely oversees suggested changes, additions, or repeals to the laws. They also instruct the prosecutors and constabulary on what laws the Crown wishes to have emphasized for enforcement, or alternately, at least partially ignored. Other than the Empress, the Lord High Magistrate has the most influence in keeping order throughout the land.
There is one other aspect of the job is that the Lord High Magistrate has the power to claim jurisdiction over any case going to trial anywhere in the country, regardless of who the accused might be. In one particularly memorable instance, the Lord High Magistrate held court when King Manfriedreich IV, Queen Reichsha’s father, was accused by the Royal Bureaucratic Council of treason. The entire episode was a power play orchestrated by the then Lord High Chamberlain. After the plot was exposed by the Lord High Magistrate, and the conspirators properly brought to their own justice, the job of Lord High Chamberlain was reduced in rank to Lord Chamberlain, until Reichsha took over as regent. What could have been an extremely nasty civil war was neatly avoided by the sage wisdom and calm political machinations of the Lord High Magistrate.
The other primary job of the Lord High Magistrate is serving as the Crown’s primary legal counsel. After all, nobody else is as qualified to navigate the laws as the person that decides which laws go in the books.