Under your nails, in your hair, the Slivers are everywhere
Sliver describes not only the name of the organization, but it's reputation and how it operates. Sliver is at its heart a criminal organization, devoted to spreading chaos and taking things for their own. They operate in violent and unpredictable ways, often taking a more extreme route to their goals than might be necessary. Their leader is strange individual, rumored to be from some place far away. It is said that he wields a pair of daggers that are so jagged and toothy that disease on any of his victims is guaranteed, for the flesh can never heal together properly. Not much is known about him, but he is one of the most wanted men in The Empire of Vin. They typically only do minor crimes, but they do them in ways that give them infamy. For example they may pickpocket, but they'll do it such that their mark gets poisoned along the way, or they'll replace all the coins with fakes marked with the Sliver's sigil.
There isn't much structure to this group. They merely function as a loose collective, sometimes in concert, but mostly on their own. The leader demands tribute from their hauls and occasionally gives them commands, but otherwise lets them run rampant. This loose structure ensures that no one truly knows everything about the guild and it's made it tremendously difficult for the Empire's forces to track them down.
They have numerous hideouts, seedy taverns, smokehouses, bad restaurants that they use to hole up in when the law is in pursuit. They don't have many allies, but everyone has a price. When given a choice between aiding the Empire and taking a few coins to help a criminal, most will usually side with the less violent of the two.