The Thieves Guild in charge of The Silver Underground of Argentis. Led by a Shadow-Walker who simply is called "The Midnight Sun", the Sterling Syndicate is not your average thieves guild due to its strict rules and its bustling headquarters that are available to even outside criminals and smugglers... as long as they exclusively do business with criminals in the Silver Underground, rather than with any smaller, younger criminal organization on the city surface.
Nonetheless, while the current Sterling Syndicate keeps most of their business confined to their underground hideout and their artificial cave system, due to their agreement with The Assembly, many of its members do exploit some loopholes in the agreement, with more extreme members luring people underground to get rid of them without a way for the guard to intervene, or robbing people near an entrance to the Silver Underground, and then entering it once its done, waiting for things to blow over. This comes with its own sort of risks, however, as too many young criminals who abuse loopholes such as this may gain the ire of the Midnight Sun due to their recklessness.
While the Sterling Syndicate mostly just cares about themselves and their business, they nonetheless have an odd sense of honor and loyalty to the city itself; not to be confused with loyalty to the argentian government, of course. Nonetheless, it has been responsible for the funneling of outsider information to the government and argentian military, whenever a threat to the city has become apparent.
Some members of the Syndicate, including even the Midnight Sun themselves, also have some ties to the Liwilai Adventurer's Guild... as sometimes even the Sterling Syndicate may recruit the occasional adventuring mercenary, if they believe that their goals may align. After all, their limited diplomatic immunity may even come in handy, even if the syndicate needs adventurers for a more benign, aboveground job that only tangentially connects to its more illicit operations.
The Midnight Sun however is sometimes known to personally hire adventurers that caught their eye, generally for more personal jobs that have nothing to do with the syndicate. These jobs, however, are only between them and the guild.