The Arm protect the people from the monsters outside civilization. Provided they follow the rules.
the City of Light
Citizens in the City of Light must take a test every year of living in the city. If they fail, then they are banished from the city. This law was enacted after the Rising (when all became deadless, the monster situation became out of hand and the secure cities were overflowing with refugees) to prevent over population.
What the test entails changes seemly on the whims of the Ever-King. Sometimes it is a test of sanity, other times for treason or even a quest must be completed.
The Arm are led by the Ever-King, the ruler of the City of Light and considered the most powerful man in the land. He has protected the city for hundreds of years.
He acts as a sort of phylactery for the city and its magical defences. The city is alive and must be powered by a strong soul.