Adeptus Arbites

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The Adeptus Arbites is the galactic police force of the Holy Empire, responsible for enforcing Imperial Law on all Imperial-controlled worlds.
While every Imperial planet has its own local police forces and its own local laws, the Arbites are responsible for enforcing the broader laws of the Imperium, and ensuring Imperial planetary governors do not plot against Imperial rule, and that all Imperial laws and decrees are followed. The enforcement of a planet's local laws is not the concern of the Adeptus Arbites. These are left to the local law enforcement agencies.

The Adeptus Arbites also serves to keep planetary governors in check; if an Imperial governor decides to skip his tithes for a year or ignore requests for troops from an embattled neighbor, one glance at the armored courthouse of the Adeptus Arbites is usually more than enough to make him reconsider.
It is also the justice court of the empire.