Like the Assassins Guild the Imperial Inquisition is an ancient and venerable institution. Unlike the assassins, the inquisitors are relatively well known, their stories being spread in books and plays, and their intervention being at times extremely visible.
The inquisitors are an institution constantly searching for the internal enemies of the Empire. From the first whiff of treachery, they will follow the trail until they find the ultimate culprit. They concern themselves with the big picture. Treacherous dukes, rebellious general, powerful cults and rogue demon summoners. If they are plotting against the emperor, it is the inquisitions duty to find them, and remove the danger. This may be by the way of the Assassins Guild, but at other times the problem is too great to be handled by the death of a single man, or the Inquisition wishes to make an example.
For this reason, the imperial inquisitors have wide reaching powers, giving them authority to command any imperial servant or citizen, and allowing them to requisition forces and supplies from the Empire's reserves. The authority of the inquisitors derive from the highest echelons of the imperial bureaucracy, and they are to be obeyed unquestioningly. Inquisitors can command assassins, imperial mages and imperial bureaucrats in the hunt for their prey, and send out entire armies if necessary.
But most of the time the inquisitors won't call out armies. While they have massive authority, they are still accountable to the Emperor. Misuse of the powers given to them may see them dismissed from duty, imprisoned or even executed. Most inquisitors carefully weigh the needed force against not only the target's resistance, but also what the Empire can spare. Sometimes they go as far as hiring freelancers, the quintessential "adventurers".
Unfortunately, with the decline of the Empire the inquisition has run into problems. More and more, individuals are finding their way into the institution that serve not the Empire, but themselves. Ranging from swindlers who want to fill their own pockets to outright traitors, seeking to bring down the Empire from within, these inquisitors with false allegiances pose a serious problem. Not only does their acts themselves cause trouble, but their very existence brings into doubt the intentions of every other inquisitor. These days, the nobles and imperial bureaucrats do not instantly spring to an inquisitors aid, for there are too many stories of false inquisitors. Though this hesitance can always be dispelled by the inquisitor calling down higher authority, the time this takes may cost the Empire dearly.