ADEPTUS ARBITES
The Adeptus Arbites is the Imperial Adepta that serves as the galactic police force of the Imperium of Man, responsible for enforcing Imperial Law (the Lex Imperialis) on all Imperial-controlled worlds. They have been granted the right by the High Lords of Terra to serve as judge, jury and executioner to any Imperial citizen they discover having broken Imperial law and the concept of due process is not one that generally applies in the Imperium. However, in those cases where Imperial law is unclear or the law breaker has committed a truly heinous deed or is of unusually high-ranking stature, the offended will be taken back to an Arbites Precinct House for judgement by an Arbites Judge, the sentence to be carried out immediately. Imperial justice is swift and sure, but not always just.
Only through constant watch and the execution of brutal law can the Imperium survive, and it is the Adeptus Arbites who carry out this function. While the armies of the Astra Militarum struggle to hold back the aggression of alien empires and protect the worlds of Mankind from without, the Adeptus Arbites roots out rebels, recidivists, and threats to the stability of the Imperium from within. They operate as they best see fit, using their greater training and weaponry to tackle foes that might be beyond the scope of local Enforcers, or in many cases to deal with a planetary government which has itself become corrupted. Remorseless and single-minded, Arbitrators do not forgive or forget any crime, and pursue their quarry relentlessly until the Emperor's Justice has been served.
The laws of the Imperium of Man are a complex web of tradition, obligation, and local custom. Thus, they can vary from world to world or sector to sector, with each Planetary Governor, local prefect, or headman pronouncing his own laws. Arbitrators care little for such trivialities, and enforce the serious crimes against the Imperium while leaving such petty matters to local officials. Murder or theft, for example, are considered inconsequential unless they affect the Imperium directly, are perpetrated against Imperial officials, or somehow fundamentally threaten a world’s security or safety. Arbitrators have little pity or compassion for the transgressions of Imperial citizens. Years of dealing with the scum of the galaxy has made many of them cynical and bitter. Others have become religious zealots, seeing the Emperor's divine hand in the letter of the law and only death is deserved for those who would break it.
Their lives are often at risk, and not just from their criminal quarry. During the course of an investigation all manner of things might come to light, including the secrets of powerful individuals. In these cases an Arbitrator can face more danger from supposedly loyal allies than the scum he is pursuing.