The Ghostly Hands

Through unique application of Imperial laws, the Emperor is currently exempt from any legal attempts at him- this exemption was originally drafted for the popularly elected tribunal meant to advise the consuls and senate on the wants of the poor, in order to prevent said tribunal from becoming so bogged down in lawsuits that they couldn't perform their job. The Emperor has been elected tribunal every year for the past three centuries.
The Ghostly Hands are agents of the Emperor and execute his will directly. A law was passed making soldiers and magistrates immune to civil prosecution provided they had received orders compelling them to act, and the person giving the orders would then be accountable. Since the Emperor isn't accountable, those few who report directly to him effectively have legal immunity for all of their activities within the Empire, and their activities range from spying to kidnapping, torture, assassination, and scare tactics. The Hands are often chosen from legionnaires and undergo rigorous training and meditation with the Emperor himself, ensuring that they know his will and may execute it in all things. The only thing keeping the senate from passing new laws is the fact that the Ghostly Hands, for all their terrible actions, are more an open secret than an official branch of the government. As a secret police  they're difficult to pin down, and most senators prefer to maneuver their colleagues into earning the Hands' wrath than dispense with such a valuable tool. The Ghostly Hands are also the only real check on the Garden of Night Blooming Flowers, and the two assassins' guilds have waged a shadow war for centuries, though of course such conflict is only the stuff of legends.
A Ghostly Hand is given a very specific and enchanted tattoo, to be shown to any magistrate or legionnaire when they try to apprehend one. Though this has not stopped some from attempting vigilante justice, itself a difficult proposition given the Ghostly Hands' training and magical equipment, the retribution from fellow Hands has been gruesomely legendary.