The Ephesian Schism
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.