Thieves Guild

The Thieves Guild is a known entity, though it operates in secrecy from the public eye. The fundamental goal of the guild is to create a community focused entirely on one goal: make money by taking other people's stuff. Joining the guild isn't simple, usually requiring one of its members to seek you out or sponsor your entry to the guild. It's believed that the guild doesn't hold to the typical hierarchies and bureaucracies other guild do. They likely operate as pockets of organization, perhaps with an oligarchic coalition of its most prominent members.
The guild has wide spanning connections to various criminal organizations throughout the world, and counts among its ranks the world's best smugglers, lockpicks, pickpockets, and deceivers. They are the Bankers Guild's greatest enemy, and the guilds greatest goal has always been to break into the Bankers Guild's gold reserve. Some attempts have been made, so far all failed.

Send your prayers to silent night,
to half-lidded eyes and flickering sight.
In padded step and whispered word,
the prowler prowls their silent flight.

Whilst weary sleep,
warm in bed,
or cold with dread,
the scoundrel seeks.

In deft, daring, dexterous sleight,
the thief takes, their fiendish delight.

"The Thief", a common children's rhyme.