Thieves Guild

Attempting to infiltrate thieves’ well-trapped guild halls is notoriously a death sentence, and there are scores of illegal thieves’ guilds within the city, big and small. Perilously few of them can be trusted.
Guilds often specialize, with one guild marshaling scores of urchin pickpockets, another specializing in challenging assassinations, a third in smuggling goods through the swamps, and so forth. Guild hall locations vary: some thieves may have elite social clubs where blackmail secrets, rare wines, and rarer forms of poison are bartered or traded, while others gather in a repulsive sewer, debating which noble’s house to ransack at the new moon. Many guilds maintain specific territories, and woe betide any thief who picks a pocket in another guild’s territory without giving them a cut. 
Thieves and smugglers might be of any age. Orphaned children are apprenticed to thieving schools who train them — often brutally — how to be incredibly effective in crime. Ancient, toothless beggars still pick pockets, pass rumors, and act as nearly invisible spies. Almost anyone you meet might have allegiances within a thieves’ guild, and gang wars between them over real or imagined slights aren’t unknown. Every decade or two a Lord of Thieves brutally carves their way to the top of the heap and makes all other guilds pay obeisance to them; then someone takes them down in a bloody fight, and the scramble for power begins again. The City Watch frankly prefers the organization and understanding that comes with a Lord of Thieves, because at least that way taxes get paid and fewer people actually get hurt.

We own the streets and the night.