My secret society is called the "Brotherhood of Night" and consists of a group of men and women who specialize in theft and occasionally assassination. Their secret headquarters is underground and they use the vast sewer systems to travel and transport their stolen goods, with several access points (such as the tavern in my creation) to the main part of the city.
Their rules are quite simple.
1. You are free to steal from anybody and everybody, except other Brotherhood of Night members.
2. Don't get caught.