The Thieves Guild
The Thieves Guild of New Genesis is a massive, many-tentacled organization, with cells and enforcers throughout the known world. The true scope of the Guild is nearly impossible to uncover, as it actively obscures itself behind fake names, false businesses, hiding in plain sight, other guilds' identities, anonymity, forgery, murder, and tongueless secrets above all else. Almost no one in the Guild is solely a member of the guild... each member is also a merchant, a noble, a clergyman, a dockworker, or some other 'primary' identity. This allows its members to continue to live in the daylight of society and have plausible deniability against any accusations of illicit behavior. Although the organization has the rather generic and monolithic name of "The Thieves Guild", this is also by design. The moniker is so broad and non-descriptive as to be useless to any who attempt to define it or hold it accountable. If one person, or even a cell, get arrested, the rest of the organization is well insulated.
The Guild is both highly secretive and highly cellular, which can cause unique logistical challenges, such as two cells working at cross purposes or taking the same 'job'. More often than not, two members of the Guild will pass on the street with neither having any inkling that they've brushed past another member. This anonymity is an amazing camouflage against law enforcement, but it means that the lieutenants in charge of coordinating jobs must take care to get the word out, often by Thieves Cant and word-of-mouth, to the general populace and 'underground'. This loose, often invisible structure is generally broken up by geographic region... sometimes by street, neighborhood, or city, each with a 'boss' that has some idea of who is 'above' and 'below' them in the grander organization. Sometimes this familiarity is as vague as a nickname or 'street name'. The 'cut' of illicit profits trickle upward, and, as long as dues have been paid, 'protection' is provided to the underlings and lieutenants (if convenient to the Guild). Rumors swirl about who is the true leader of the Thieves Guild. Various movers and shakers of the known world are suspected, including kings, merchant lords, various aristocrats, pirates, even high priests and some non-humanoid creatures. A common and persistent rumor is that a long-retired adventurer is at the center of the web.
On it's face, the Thieves Guild provides 'opportunities' that exist outside of the boundaries of the law. Have a problem the city guard can't or won't help with, like a bad neighbor, a dishonest business partner, or the street thugs in your neighborhood ? Need something that is banned or hard to find? Need to deal with a cheating spouse? The Guild will provide solutions to any and all of these problems, for a price, of course. Occasionally, the Guild (or at least more visible member of it) will even make a well publicized donation to an altruistic organization (via an intentionally transparent cover), like an orphanage or school, just to keep in the good graces of the general population. Lawmakers 'on the take' often deny the existence of the Guild outright.
In a sense, all wealth in the world belongs to the Thieves Guild... they just may not have decided to take it yet. It would not be unreasonable to compare the wealth, resources, and manpower of the entire Thieves Guild to rival that of a nation in the known world. Of course, the Guild's ability to gather and focus these assets is nearly impossible on such a scale, but city-sized efforts are well within a strong Boss's abilities. The gold piece value of wealth owned, or available to, the Guild is immeasurable. The same applies to weapons, armor, and even magic items. Depending on the level of the Boss, they have access to almost anything they might want... because, of course, if they don't already own it, it's only a job away. Alchemical items are a specialty of the Thieves Guild and they are easily the most reliable source of poisons in the known world. Specialty tools, such as Thieves Tools, poison maker kits, Breaking and Entering kits, etc, are almost exclusively available via the Guild. Illicit drugs, even rare ethnic inebriants (such as elven Elverquisst and Evermead, dwarven Underwine, etc) can also be sourced from the Guild.
Thievery may not be the world's oldest profession, but it probably comes in as a close second. The first whispers of a 'Guild of Thieves' first appear with the first civilized Human settlements, around 500 NG. Likely a conglomeration of brigand groups united by a strong leader, the Guild has existed in some form for centuries. Detailed records on the going's on of the Guild are intentionally rare and perhaps non-existent. Legendary heists, major leadership shake-ups, and big busts by the law tend to spread like wildfire via word of mouth and linger in a oral tradition. These stories, of course, evolve over time, becoming folk tales that are used to express a lesson or moral to members. 'Snitching is repugnant', 'Pay your Dues', 'Don't double-cross your peers (unless they deserve it and you can elegantly get away with it)' are common themes in these Guild 'historical' tales. Like a cancer, the Guild has grown with civilization over the millennia, always existing as a shadowy parallel to mainstream culture. As the Guild would tell it, every exploratory ship sailing to new, unknown lands, had at least one Guild member on board, every magical or technological discovery was noted by the Guild, every war or conflict was profited by, and every secret, no matter how minor or cosmic, has been shared via Thieves Cant. The history of the Thieves Guild is the history of the known world.